Do you use it on a hard surface? or like most people your knee on a cushion or on your bed?
If its the latter, then it is more than likely to be the thermal mod overheating. Fibres get clogged in the fan preventing it from cooling the processor, causing the intermitant crashing.
The solution is to send it to me for repair and no it wont be expensive to fix. If you leave it and the mod burns out you could potentially lose all your data.
For more help call me on 0161 888 1232 or look at the website http:\\click here