Computer turns on and off instantly

Hi folks

Been a while since I posted. So long that I forgot my username and had to re-register! Anyway this is what's happening to my computer: I press the power button, turns on for a split second (lights + fans running) but shuts off right after (no system beep). I searched the forums for this problem and found similar problems, but not exactly the same as this. If I keep pressing the power button it is likely to do the same thing but theres a chance the system will boot as normal. If I hold the power button, the system will repeatedly turn on and off, and eventually (after a random number of tries) the computer will boot up. Once it boots, the computer will run as smooth as ever and won't turn off by itself. I'm 80% sure it's the PSU, however, I'm hesitant to go out and buy a replacement because the current PSU is an Antec TruePower 480W. I presume Antec is a very respectable brand (correct me if I'm wrong) and it is still under their 3yr warranty period). I did not expect this to break down before 2 years time so I just want to make sure it really is the PSU before I buy a replacement. Maybe I should try another brand? I also checked Antec's warranty online and S+H is not covered for warranties so I figured if the PSU is faulty it would be wiser to just buy a ~$50 replacement.

TIA