Well something similar is happening to me... Sometimes when I back to my PC it has restarted (?) and when showing some videos gives me a bluescreen related to atipmdag.sys (when Windows 7 drivers, not ATI -read the end to understand why and under what circunstances) .
Also, it doesn´t render well some areas (ramdomly) of the screen:
I have an ATI Radeon HD 4870 x2 with NO ccc (control panel) installed (only the drivers) and Windows 7 b7048 64bit with 4 Gb RAM
PD. If I install the Windows 7 drivers the screen bug solves but the "atipmdag.sys" appears.
I have thought it could be something related to the Windows 7 version is a beta (I´ll try to install a RTM in a week or so to see if it solves)