Riva framebuffer support is also called "rivafb".
Rivafb is a framebuffer driver for nvidia cards made using xfree86 sources and leaked nvidia stuff. It wasn't made by nvidia. Because the programmers who made rivafb don't know everything about nvidia's cards it is very, very buggy and unstable.
Normally it already doesn't function stable and without problems but it is even worse with nvidia's drivers.
Replace rivafb with vesafb if you want any framebuffer. That's the only solution.