I've posted about this issue before and thought I had a solution but it has started to occur again. My pc locks up during 3D applications constantly. It's a hard lock (no mouse and keyboard use) until I physically reset the machine. I've replaced my motherboard, RAM, Video card, and processor. I've tested another power supply and all the components work fine. I've run Memtest for over 36 hours per stick (2x) of RAM and they have also turned out fine. I even purchased new RAM just to be sure. My PC still locks up in 3D applications. The funny thing is there are no logs of any errors after reboot. No crash report, no video card driver failure, nothing. System specs are below: Asus P5N-D Mobo Intel Q9300 Quad-Core 4gig Cosair XMS2 RAM (2x sticks) Cosair 620 Watt PS Nvidia 9800GT (Tried an ATI 1950X as well) Windows 7 Ultimate (RC) (Installed this after the same issue kept occuring in Windows XP Professional) No other devices or cards in the box. Can anyone help? **edit I forgot to mention that this only happens with 4gig of RAM installed. I've run for days on 2gig (1x stick) and haven't encountered an error. I have tried each RAM slot independently, as well as in multiple configurations.