Paul, if underclocking your CPU is fixing a problem then your fan isn't up to the job as blue screening because of the CPU means it's overheating. I used to get this a lot with an old pc I had that I overclocked. I found out the hard way

You may have a faulty board, CPU or hard disk if it still does it when you've underclocked and wiped down.

Out of interest, what type of CPU are you running?