Having problem with PC

[worldsenvy]

I am using an Intel 865gbf motherboard. My PSU is the Ultra X-connect 500 W powersupply. When i turn on the psu switch at the back, the comp comes on (fans and lights, evrything) for a second without me hitting the power button in front and then it all goes off. Before i had a thermaltake psu and there was the same problem.
The real problem is that sometimes, randomly, not necessarily in the middle of a program or anything, my Pc just turns off. I have an CPU temp alarm and it never goes off before these sudden switch offs. The computer turns on after a while sometimes. I have checked the thermal grease and everything. This has been happening with both PSU's. Then fnnaly my thermatake stopped working after one of these so i bought the ULTRA but it still goes off. Could anyone help me out here?

Thanks

 

[montag451]

Hold on - - So this problem, it happens intermittently, OR are you stuck with a completely non booting sys?

 

[worldsenvy]

wel.. it was intrmittent and all i had to do was switch the psu o an off and it would work again. But then the Thermaltake stopped working forever, and now with the ULTRA i ned to switch it on and off a few times, pray and then it comes off after a while(=between 5 mins and 2 hours)... though im sure thats not good for any part of my computer.