Hi Adrian,
As @danj said, you most likely will need a new logic board. If you can get Apple to fix it for free or relatively cheap, go that route! However, if you are unable to get the help you need from them, we sell 3 different models of logic board for your particular machine, depending on which sub model you have. They aren't cheap, but they will fix the problem:
[product|IF123-050|2.7 Ghz Logic Board]
[product|IF123-051|2.9 Ghz Logic Board]
[product|IF123-096|3.1 Ghz Logic Board]
Good luck with the repair!
- Jasen