What can the fault be with constant running on lynx 2 combi boiler?

I have a Potterton Lynx 2 combi boiler that has had its fair share of problems over the years.It now fires continuously with the result the pressure rises, the prv release a bit and then lasts a couple more days before it hits low pressure and needs topping up. The gas board have replaced the prv , the pcb, the disc thermostat and the thermister , the gas valve , all to no avail .It works until there is a pressure build up .I do not think the PCB is at fault and think the culprit is a faulty thermostat .Any suggestions would be welcome