Is there anything I can do to help you figure out the cause of this? Debug logs, system info, trying various settings, etc.? I'm no codesmith, but I have a fairly good understanding of the mechanics of bugfixing.
I've actually got two other notable issues I'm experiencing. One is that there is an incredible slowdown when the yellow circular explosions are being rendered onscreen (from bombs or capital ships exploding, etc.). I am using the regular vp_effects file, so I'm not sure what the problem might be. Other effects do not seem to be an issue.
Finally, I have an issue with sound. I used to get really bad stuttering when sounds were being loaded, so I added the snd_preload parameter. Now, however, it seems that sounds don't get taken out of memory after they are loaded, and eventually there is no room for new sounds. This means that if I play more than a few missions in a row, new sounds slowly stop playing, so I don't hear any voices, etc.