Ok, so this is basically a "me too" post - just so you know there are more people out there with the same problems.
There was a couple of threads a month or two ago describing exactly the same thing. (I did a search but I couldn't find it again.) They also exhausted most possibilites and couldn't come up with a solution.
I have a SiS735 (ECS) and a KT266A (MSI) board. Both running RH7.3 with latest RH kernels. A Gainward GF4 Ti 4200 will run without problems on the KT266A, but will flicker and hang on the SiS735 every time. Both a Quadro2 and a GF2 MX card runs without a hint of a problem. Using WinXP everything also runs fine.
The hang will occur whether I run 3D applications or not - it merely increases the chance it'll happen sooner.
And yes I've gone through all the AGP settings, BIOS settings, driver versions, kernel versions etc. etc, but to no avail.
I have a chance borrowing a Quadro4 card next week - just to try...