Currently Being ModeratedJan 20, 2013 9:22 AM (in response to peesemould)
It's possible that the 'bad hard drive' diagnosis was too hasty or downright wrong. It's possible it could be your logic board was on the way out and is now gone, especially if the usual suspects (PRAM, SMC, HD) haven't made a difference.
I would take it back to the Apple Store and tell them the full story. Ask if it could be a logic board problem (e.g. failing capacitors).