Just downloaded the 5336 drivers - no change.
glxgears came up with an interesing new message the first time I ran it:
glxgears: relocation error: glxgears: symbol gettimeofday, version GLIBC_2.0 not defined in file libc.so.6 with link time reference
It crashed the box te next time.
I'll run memtest86 today and see what it reckons about my RAM.