And check out the web sites for the 9500 for information about card distribution. PCI PowerMacs can be very fussy about what card works where. The most important card in your case, of course, is the power hungry Radeon video when you choose to fit it.

Also make sure that you have the latest MachSpeed helper control panel for the accelerator. It worked pretty well in my 9600 (G4/450) but didn't solve all problems (PhotoShop corrupts files but I don't use the machine for PhotoShop so I haven't investigated further; probably Level 2 cache related).

Have you tried running with 100% matched memory, fewer cards and devices, just to identify a stable set up? And, although it sounds obvious, reverse the overclocking.