I'm going a bit off-topic here:
Here are my findings so far. My PS3 has shut itself down a couple of times now. After the first time I let it cool down, cleaned all the vents thoroughly (although they weren't clogged or anything). It fires up normally. Then it states the system hasn't been shut down correctly and it does it's file system check. After that I'm back to normal and running the XMB. Everything works.
Then I boot up GT6 and I can play without problems for about two hours. Then all problems start. The last couple of times it failed were all between 2:00 and 2:15 hours of firing up the system. I tried adding more cooling capacity by running a big table fan in front of the air intake. That didn't make a difference. Then I tried running the system with the front half of the casing disassembled and blowing cool air directly to the hardware with the table fan. Also no difference, after 2 hours it shut itself down again.
I also got rid of most of the dust in the top half of the system. The bottom half, where the fan and the motherboard are, are inaccessible unless you want to take the motherboard out, which I rather not do.
Further I tested the power supply on its own. It delivers 5,04 on the 5 V line and 11,98 on the 12 V line, so that should be fine as well.
I don't think it's the famous YLOD problem where the motherboard is broken, because most systems then become unbootable after the problem has started. After mine is cooled down I've been able to play for 2 hours minimum.
I still suspect the power supply, as it is a weak point in the old PS3 60 GB design. It is uncooled and has very little air gaps. There have been cases where a new power supply has solved problems.
Any thoughts?