It could be really anything, CPU, PCH or even the hard drive. If you have a spare drive or can borrow one that would be one of the first things to try in my opinion. There’s a number of Macs of that series that give problem with PCH but it’s not terribly widespread and in any case a replacement wouldn’t make much sense economically. Used C2D boards come quite cheap nowadays but I would still verify the hard disk to start with. Anything unusual noticed before the issue showed up ?