Repeated starting and stopping - power up / power down is symptomatic of a power supply fault - easy to replace - a small computer repair shop may sell you a second-hand PSU at a generous discount. Do not be frightened of using a lower Wattage PSU unless you have added lots of peripherals to your machine. I replaced a broken 350W PSU with a second-hand 250W (£20) without problem.
Good luck...
Peccavi, July 2010