Here's something interesting: I can reproduce this on regular Doom 2 maps, but I can't seem to reproduce it on TUTNT: Supportive Edition. I can also reproduce this if I start a game (MAP01 of D2), open the main menu, and the hold up/down arrow. I also have vid_fps set to true so I can monitor the delay. It occasionally flickers as described in the OP. This seems to stop happening if I turn VSync off. Tried with a fresh INI, but the problem persisted.
It's like in the video in this thread, but the delay is much smaller:
viewtopic.php?f=50&t=65552Also, I did some tests while I was writing this post and it turns out this does happen on 4.1.3, but I reported it as "not happening" because I haven't played Doom 2 to notice it. My bad. I checked my Nvidia control panel, but most settings are "Application-controlled" or "Off". Only "Shader cache" and "Texture filtering - trilinear optimization" are set to "On".