I recently repaired an older Gateway Laptop MT6451 that had died on me. The system has a built in ATI Radeon 1150 Express chip with the most up to date drivers I can find. I first replaced the cpu without luck and then replaced the mobo. I got the laptop up and running. However, shortly afterwards the display shut off.
While it was still running I hooked it up to an external monitor and got a display there. After some fiddling back and forth, I got it so, it would give a dsplay from a cold boot, but wouldn't display on a warm reboot. I ended up reseating the cpu and adding more heat sink paste. I then booted up, got a display and left the laptop running for 24hrs. It was still running, still displayed and hadn't locked up. I rebooted and again on warm reboot no display.
If I leave to cool it will reboot with display. I'm thinking heat is causing something inside to expand enough to disconnect the built in display but really have no idea why.