Anotyher thing to consider is the memory. Now I know you are going to say that it is alright but it may be that the crashes happen when the higher memory blocks are used as with more porocessor power and so it would be worth testing with Memtest86+ properly. Like these instructions:
I would suggest that you download and run the latest version of Memtest86+ to check your RAM first.
Memtest86+
http://www.memtest.org/Insert
one stick of memory in
slot 1 and run Memtest on it for at least
10 complete loops/cycles and if there are no errors then swap it over with the next one and continue untill you have checked all modules.
If you have any errors the module is faulty.
If you have any faulty modules you will have to return the whole kit as they are matched.
Post back when you have done that with the results.