I just got a Radeon 8500 with 3.6 ns memory from hynix. Sadly, I seem to have got
very bad memory chips, as they freak out already at the default speed of 275 MHz

Most notably in the Nature demo and especially in the Vertex shader demo, but also a bit in some other demos. After under-clocking the memory to 270 MHz, most of the artifacts were gone, and only after under-clocking it to 265 MHz, they were completely gone.

Geez... what's up with this? 3.6 ns memory is supposed to run at 275 MHz memory (which is the default speed on the Radeon 8500) without any problems, but apparently not here...
Any ideas what I could do here?