From some time ago, I've been experiencing some strange issues while playing Open Arena. The following lines from the article "Gaming with the open source R500 driver" struck me for they describe exactly my problem:

[...] the OpenArena game [...] had successfully started up and we were able to play the game initially as well as run a timed demo, but after a while, the frame-rate would drop substantially and render the game inoperable. This is likely due to a video memory (GART) issue.
I have a radeon mobility 9600 and it is quite capable of giving me the old good 125 FPS with the oss driver, but after a while (sometimes very quickly) it drops as low as ~35 in the worst cases, and usually it just keeps jumping between 85 and 125. Sometimes restarting the game (or the server) helps for another while, sometimes it doesn't. The funny thing is that it happens too with fglrx, although it usually stands up for much longer.

If somebody could give me any hint as to what to look at to solve it or to submit a meaningful bug report I'd be grateful.