I have a Hotpoint TDC60 (condenser) tumble dryer where the heating seems to work intermittently. It will heat up for a few minutes when I first start the tumble dryer, and then suddenly go cold (even though the drum keeps turning). If I turn it off and turn it on again immediately then the drum turns but the heater doesn't work at all. But if I leave it for a few minutes before starting it again then I get a few minutes more heat before it goes cold yet again. It's a very slow way to dry clothes! I've swapped both the thermostats for new ones but this has had no effect.
Bingo. I read your question a few weeks ago when I had the same problem. Today I fixed it and it's quire easy. Take the lid off and there's a PCB in a plastic housing. The soldered joints on the middle relay were dry and if the PCB is tapped the heater flicks off and on. Take the PCB out and resolder all the relay connections and it will work again like mine. DON'T FORGET TO TURN THE POWER OFF - OBVIOUSLY or if in doubt get someone in that can do that sort of stuff.
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Try fixfault.com to fix tumble dryers.