Okay, Problem solved. Just bought me a new mainboard,new cpu, new ram.
I tried everything to fix the problem, it didn´t help.But it was denfinitely some kind of hardware malfunction.Damn, I really had no time to check any good offers.And the worst thing is, I didn´t enjoy buying the new hardware, which I normally do.
Actually I wanted to buy an Intel CPU (after three Athlon in five years) but my dealer vended only P4 which were too expensive. So I ended up in buying a Sempron 3000+ (almost at the same price as my Athlon 2500, which I bought 2 years ago!)
Anyway, thanks for your help