A weird problem: when booting the system, nothing appears. I mean, the monitors get no input. I can hear the hard disks whirring and the keyboard leds light up, so I suppose it is the videocard. This is the second time in about a week. I've rebooted several times to no avail.
In the end, while it was on, I jiggled one of the DVI ports on the side of the videocard. The machine rebooted and I saw the BIOS loading screen again. Everything proceeded as normal from there on.
Update 16 sep 2012: exactly the same problem, exactly the same solution.
Update 3 nov 2012: same problem, above solution doesn't work anymore. I disconnected the monitors from the videocard, opened up the case and tightened the screws on the videocard. I also vacuumed the rear fan and the cpu fan. After that it booted up again.
Update 23 dec 2012: same problem, jiggeling the DVI port worked again.
Update 23 jan 2013: same problem, had to open up and re-tighten the screws again.
Update 4 apr 2013: after a couple of freezes at different moments, had to open up and re-tighten the screws again.
Update 12 apr 2014: same problem, jiggeling the DVI port worked again.