Re: FX 5200 + Debian Etch won't work for me.
0) Setting NvAGP to 0 disables AGP transfer rates, and falls back to PCI rates. This would detemine whether the problem is an AGP driver, BIOS or hardware issue, or a problem elsewhere.
1) Yes, you need Composite to use Beryl. However, if the problem isn't present without Composite (and/or Beryl), then it could suggest a bug in X, or Beryl. Or it could still be a driver bug. Regardless, its a useful isolation test.
3) I wasn't advocating removing the current kernel, just adding another kernel for testing purposes.
You'll need to perform all of the tests I've requested to better isolate the cause of the problem. Its quite possible that they won't all be needed if the problem is eliminated after only performing some of them.