hmmm, maybe its not much related to the problem but worth of checking

someone mentored to check if in the bios u don't have turned options to decrease clocks of cpu when its overheating, the best u can do is to turn that function completely off

second thing is, i find the newest graphic drivers not really compatible with q3, aside of course q3 is really sensitive for every other process that eating cpu time, a good option to get rid all those lagging because nowadays computers run so much shit in the background is to use a software like "process lasso" where u can set all process to use core0 and lower down their priority to below normal, then set for quake3.exe affinity for core1 and priority realtime, while all other process uses core0 quake can use the same time core1 with the highest priority and this not going to make impact on the pc performance in general since 99% of the process don't even need a full one core potential but they affect q3 when they uses the same cores, just remember to turn the balancer off for quake3.exe or completely turn it off since we only want assign basic process affinity and priorities. U can of course set the main process to have affinity for core0,1,2 and for q3 only the core4 since q3 tends to make weird lag when it works trough multicore alegoritms

short question, you got a newer ati gfx?