So far so good after the installation of the new PSU. 2 hours in PCars, everything is ok.
Before the new PSU I had coil whine from GPU and I thought it was the typical coil whine most AMD GPUs have. But now it's gone. It seems that there was not enough power and I don't think the multiple rails work like 17A*4= 68A. There was surely big loss of power supply not only because of the 8yrs old PSU.
Right now I have again the q6600 overclocked but at 2.8 (9x311 FSB) and the RAM slightly underclocked to achieve the 1333 FSB P5KC limit (DDR400 here). Idles at 32 degrees. Vcore is 1.35 and goes up to 1.37.
Will it be beneficial regarding performance to go for 3.0 or 3.2ghz? Is there a way to achieve 3.2ghz without passing the FSB motherboard limit? Bottleneck is already minimal though. This q6600 G0 revision wa really a good buy back then!
Thanks for your great help and advice guys!
Q6600 OC 3.0ghz, MSI R9 280x Gaming Edition, 7gb ram, Asus P5KC