Puma 80 overheats and shutsdown when DHW switched off?

This 8 year old Potterton Puma 80 has had first a new diverter valve fitted, and now a circuit board, but still overheats (burner remains on) and shutsdown to safety when DHW tap is switched off- but only if the boiler was previously heating the rads.
DHW works normally otherwise, but central heating sometimes won't light and switches off after a while. I think it is a little scaled up but would like to get a few more years out of it.
I have flushed the system, cured any leaks, and checked the water flow switch (which makes a circuit when a tap is opened and vice verse, and has 22.6v).
Should I try changing the pump?

James
February 2005