Memory issue with recent fglrx drivers?

This is a strange one...

SL runs perfectly with version 8.28 of the fglrx drivers. Anything after that an I will get FPS dropping to 1-2fps after a few minutes and everything becomes unusable.

If I upgrade the driver to 8.35.5, then set the SL video memory option to 64Mb (I have an ATI X700 with 128Mb) that seems to cure it.

Any idea what's causing this?

server glx vendor string: SGI
server glx version string: 1.2
client glx vendor string: ATI
client glx version string: 1.3
OpenGL vendor string: ATI Technologies Inc.
OpenGL renderer string: RADEON X700 PRO
OpenGL version string: 2.0.6400 (8.35.5)
Debian, 2.6.18, xorg 7.1 i686 1Gb RAM, P4 3.2Ghz

This thread may be pertinent:
/263/85/175959/1.html