Anyone who's looking through this thread looking for the answer to this issue, I've found what the cause is!
I've had this issue with tmodloader 64bit for a long time so I decided to look into it. A message in the cmd of 64bit tmod keeps saying:
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I researched what this was and it turned out that the game was trying to run using adaptive VSync, and your graphics card, whether you told it to or not, is trying to use some other form of VSync called "Adaptive Verical Sync".
The fix is going to your control panel, for me it's nvidia, finding your global settings (or application settings if you don't want to enable Adaptive Vertical Sync for every application), finding the vsync option, and changing the value to "Adaptive" rather than "On" or "Off".
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Hope this helped anyone who's been having this problem
EDIT: Play in fullscreen with Frame Skip: OFF