T400 with discrete graphics sometimes hangs with gray screen

[kitor]

I own T400, model with switchable graphics. On ATI one I sometimes encounter hangs - got gray screen and I can only hard reset whole notebook. It sometimes not happens for more than a month, sometimes 3 times in row the same day. On Intel I never had similar problem.
Two facts - I'm using Windows 8.0, switching graphics in BIOS (but I have the same behavior on both Win7 switchable driver, and with independent drivers when switching them from bios). Also, bending, shaking, etc changes nothing, I can literary throw it on bed from two meters and it won't crash.
Anybody got similar issue? I suspect Windows 8 as a problem (as previous owner had WinXP with ATI as default card), but I don't have two spare months to test it on Win7...