The springs holding the GPU heatsink against the chip are too weak. I discovered this after a reboot and getting the video error signal (2 long beeps, repeating), and no display on the monitor. The heatsink had flopped down somewhat and I guess that the GPU was overheating.
I removed the heatsink, applied thermal compound and remounted it. Then I laid the computer on its side so that gravity could help hold it down. It fired up normally and I used it for a few days in this horizontal position. But on standing the computer vertically the heatsink soon drooped and I got the video error signal again.
The heatsink is held in place by two plastic pegs with springs, only on two corners. I am hoping somebody has already solved the problem of mounting it more securily and can tell me how to do it. Thanks.