I've been having issues getting a solid vsync on my Win98 system which I think should easily be able to run Quake 3 at a locked frame rate:
Asrock 775i65g
Pentium Dual core e5800
512mb ram
Geforce 4 Quadro 380xgl (happens on regular Geforce 4 ti 4800 as well) 45.23 drivers

Enabling the Swap Interval option in the Quake 3 cfg file enables vsync and appears to work properly for a while; however, eventually the game will always revert to a consistent stutter. Reloading the level will usually fix it but eventually it will happen again. Forcing vsync in the Nvidia control panel for OpenGL doesn't do anything. I run the game at 640x480 with only texture resolution increased. It's not a performance problem since the game runs fine when vsync is working properly.

I've tried setting my monitor to different refresh rates like 60, 75, and 85 to see if vsync will behave differently but it doesn't change anything. I've patched the game to the latest version which did not make a difference. I thought about installing Team Arena to see if that fixes it but thought it was strange the base game had this issue.

This does not happen in other games based on the Quake 3 engine like Heavy Metal Fakk 2.

Has anyone else noticed this?