Tony's right BTW there is no pump over-run. Thats why the low speed fan is important to clear the excess end of cycle heat.
I am glad my memory is OK this time as it must be at least 6 months since I last repaired one of these and that was only the pump.
If its overheating then I would be expecting it to be the low speed fan operation which we are waiting for the OP to tell us about.
Otherwise scaling or debris on the primary heat exchanger inside surface.
Tony