I'm suddenly getting some video stuttering, but only after the computer woke up from sleep...

Does it happen after you've woken your computer up? (Assuming it was in a sleep mode)

No. I leave it on, just switch off the monitor. I don't use sleep, hibernate or similar.
OK, thanks for the suggestion. Will try it should it happen again.

It's quite normal to get stuttering for H.264 and H.265 materials when using "System Default", as it usually disables any form of 2D/3D acceleration. I've always had it set to "Enhanced Video Renderer" by default in MPC-HC and never felt the need to change it.

As to VLC, imho it's not really that issue-free in Windows, though for Linux it's still my player of choice. As to why you're getting the same issue with Pot Player, I can't say.

So after driver update, I had two days of running the PC non-stop, and the issue didn't appear. Just as I thought updating did the trick, it happened again right after restart. Which makes me thing the issue might have appeared right after restart before, only I didn't notice and thought it happens when the PC is running for a long time.

This time the stutter is pretty heavy even with OpenGL and Windows GDI when running VLC, but it still runs fine when running in Windows Media Player or MPC-HC.

Here is another thing though, (which might help diagnose the problem?) If I have a video file open in VLC (and paused), then the stutter happens too when I play another file in MPC-HC in another window.
Looks like switching to MPC-HC is the workaround/solution. I still wish I find out what's causing this.

I do have a question though, when I open a 1920x1080 video file, the window shows it in 1600x900. Increasing the window size doesn't help, the video is still 1600x900.
I made sure I zoom in to 100%. In fact even zooming to 200% keeps the window at 1600x900. Zooming to 50% makes it 960x540 (which is correctly 50% of 1920x1080).

I tried different files, including sample files downloaded from the internet.

In full screen the file is displayed correctly.

Is this something that can be changed? I couldn't find anything in Options.

Options->Player: Uncheck Remember last window position and Remember last window size.
Options->Playback: Check Auto-zoom and set it to 100%.

That should work and make every video open in its original size. Of course, because of the player controls, the seekbar, the window title and frame and last but not least the windows taskbar you can not have full 1920x1080 on a 1920x1080 desktop. It will naturally be scaled down a bit unless you go fullscreen.

Run temperature monitoring software and monitor the temperature of both your CPU and GPU. Google to find out what the maximum temperature of each should be, and your temps should be as far under that as possible. If your temps go over max, it can cause malfunction that could lead to stuttering, crashes etc.

It could vary from one program to another depending on whether the program does it all with CPU rendering, or accelerates it with the GPU, so one program might work and another might not.

For the 1920x1080 problem, this is almost always because the person who ripped/created the video extracted it from a source video which was 1920x1080, and put that in the filename, etc. - however the OUTPUT of their video transcode was to a lower resolution. Unfortunately lots of stupid people out there don't know how to properly label files and provide the correct information. Never trust what the filename of a file says it is, such as 1080p, 720p, etc. - it is always what the video player says it is when it plays it, and if it is less than you expected/anticipated, you got a bum copy.

I did that already.

No, why should I go by file name? I would notice that immediately. Those are real 1920x1080 videos.
I have those set already.

Why shouldn't it be possible to have a full 1920x1080 video (or even videos bigger than your desktop resolution) and have the seekbar... etc off screen (or have part of the video off screen). That's how it worked with VLC.

By the way, the stutter doesn't happen on "System Default" when it doesn't happen on VLC (and when it does happen it happens even on the simplest of videos).

The problem is somewhere else, not with VLC. I'm going to use MPC-HC as a workaround, but I really wish I could get to the root of the problem.

Odd, I never got that. Try messing around with the stuff in View -> Video Frame. I have "Touch Windows From Inside" selected and the "Keep Aspect Ration" option checked for example and it always scales & displays things properly.

Edit: Ok, I got what you meant, though I never had a use for it :). Try View -> Video Frame -> Normal Size and then Zoom -> 100%.

Yeah, that's probably because when using "System Default" it ends up in the same place as it would in VLC (aka it uses the same renderer). To be honest I think it's a driver issue or maybe some DirectDraw problem caused by some older codecs you might have had installed and did not get removed properly.

I'm not sure how you can get to the bottom of things though. Maybe someone with more video-codec and DirectDraw filters knowledge lurking in these forums can give you a hint.

OK looks like I was wrong. This seems to be a limitation set by Windows, because even manually I cannot make a window (any window, not necessarily MPC) bigger than screen resolution.

So it looks like screen resolution minus seekbar.. etc is the hard limit. I thought it was different in VLC, but it's the case there too.
Yes. And the problem is, I'm afraid the issue will appear somewhere too, like a video game for example. Which is why I'd really rather get it fixed.

I hope this is not a hardware problem (which would explain why it's so random). But I took out my graphics card, dusted the fans, made sure all cables are tight, and nothing worked.

Anyway, thanks for the help everyone. I'm going to mark the suggestion to use MPC-HC as solution, even though it's still not really solved, and +1 all round for taking the time to help me with this. If anyone has any other suggestions, on what I could try, please do post here.

I wouldn't worry too much about it - video games usually use their own codecs for videos and as for the renderers I guess most will render video to a 3D surface anyway, which seems to be ok in your case (the [url=https://msdn.microsoft.com/en-us/library/windows/desktop/ms694916(v=vs.85).aspx]EVR[/url] does just that if my understanding is correct).