Very Strange Things Have Happened

So hey someone, I just registered to this site because I have been having some very annoying troubles lately with my 20" 2.4ghz Imac alu. I have had it for about 3-4 ish years, I don't really remember, but thats not important. Anyway way, since around 2009 it has been shutting itself down periodically, it started at about once a week and by the beginning of 2011 it got down to once or twice a day. I then went and lived in a very remote community with only 5 or so families there for about 6 weeks, the shutting down practically stopped, it shut down once the whole time I was there. I used the computer the same amount there as I did at home if not more. After 6 weeks I returned home... BAM it started turning off about 5-6 times a day while I was trying to do school (distance ed highschool). I stopped using that computer for about 1 month really mad at it, I had been using my Moms 25" Imac alu, I then went on a trip to chine for 3 weeks, I got home, it was still turning off, when I realised that I would NEVER turn off when I played StarCraft II. I left StarCraft II runing for about 2-3weeks didn't turn off once the whole time. Today, I saw that there were some software updates to be downloaded, I downloaded them and istalled them. Now... My computer is turning off again, it has turned of 3 times in the last 9 hours..... I have tried resetting the ram cards, I have rest the SMP (I think that's what its called) by unplugging the power cable from the back many times, never with any positive effect. Has anyone even heard of such a problem??? Any advice would be greatly appreciated...
Thanks,
Chris

I own that very machine. Got it used from a close friend. Started doing it after 2 weeks. What solved it for me was to Reset the PMU and the PRAM. I did both to be sure and it's never shut down since. Been 3 months now.

I tried reseting PRAM last night, it turned off today >.< . How do I reset PMU?

Google it and find this and try it:-


Intel-based Macs: Resetting the System Management Controller (SMC)

Nope, tried both and my computer is still shutting off. I have done some other research and I read something about the power supply being the problem, does any one know how much it would cost to get a new power supply from apple? I know its not the outlet I'm using, I have tried moving it all around my house, with and with out a surge protector.