I monitored the output voltage of the PSU with a multimeter. It fell to 10.5V (instead of 12!) shortly before a crash happens. I even had a crash without the GPU strained at all! Less then 50%. I also used MSI Afterburner to increase my fan speed for certain GPU temperatures. But nothing helps. It is sunday now, and in my little corner of the world, all shops are closed. I will be ordering a new PSU online and in the meantime, I will use my old HD4870, because I hate crashes. If anyone is interested, I made a thread in the UBI forum that could help someone else with the same problem:
http://forums.ubi.com/eve/forums/a/t...5/m/9621013229