I have been using Nvidia drivers and Mplayer for a long time, and I never found this bug.
I have a standard machine, PII, intel chipset 440B, and TNT2 M64 ( perhaps your error only appears with AMD)
Have you tried to diferent AGP drivers?
In my case, NVidia's AGP is a good solution, perhaps in your case, is better use AGP-gart from kernel.
I recommend you add MTRR (Memory Type Range Registers ) when compile your Kernel, and RTC ( Real Time Clock ), and of course, choose your processor
For compile Mplayer ( I use cvs version with ffmpeg codecs ) I never had a problem with gcc-2.96 ( Redhat version )
Have you tried append "mem=nopentium" in you lilo or grub ?? ( but I think this only was for hanging with AMD )
And, perhaps, another problem, is a incompatibility with xscreensaver. (try to dissable screensaver )
If screensaver start when playing a movie, I don't know the results, but, if I start 2 mplayers, the background movie is a blue screen, and sometimes, two movies are a blue screens.
What version of mplayer are you using??