It may well be the PSU, especially if you have added hardware items such as another hard disk, DV/CD Rom drive or something similar.
Suggest you check the wattage of your PSU and get another larger one. Something around 400W +. Even if it turns out not to be the problem, a spare PSU never comes amiss and most are reasonably priced.
It might be worthwhile checking that the small front panel connectors are firmly connected and haven't come loose. Does your reset button work?
Once you have removed your case side panel, I would also check all electrical connections, including that to your motherboard.
Hope this helps.