My Hoover Condenser TC650 filter light etc?

My Hoover TC650 problem initially started with the filter light coming on, machine not heating up and cutting out. A new heater was put in plus a sort of switch. Machine is now heating up fine but after a while cuts out and stops for about 7 minutes - filter light going on and off - then restarts, runs for about another 6 minutes, cuts out again and so on. My very good repair man has been incredibly patient and has been back at least five times to try to find out exactly what is wrong. When machine starts from cold it runs for longer than the 6 minutes, assume an overheat or blockage somewhere but ...