Zanussi TCE 7124 W condensing tumble dryer - works sporadically?

I have a Zanussi condensing tumble dryer Model TCE 7124 W.
About 3 years old (heavily used).

Over a period it deteriorated. It would start drying normally, then after a while would stop (no heat, no drum rotation, nothing at all), but if I pressed the START button again, it would start.

It has now got worse. It will work for only 30 seconds, then it will stop and make a few clicking sounds. If I just leave it, it may dry in fits and starts over a long period, never for more than about 20 seconds at a time.

I can get it to work for 30 seconds easily enough by switching it off and on again – but it will only work for 30 seconds.

This is true for all programs and timer settings.

All the filters are clean, as is the condenser and, yes, I have emptied the reservoir. I have also tried cleaning the drum with vinegar, as suggested in the instruction book.

Strangely, it will sometimes work late at night or early in the morning (when it is cooler) – suggesting that the problem could be with a thermostat. I replaced this thermostat (the one just inside the door, at 12 o’clock) but it did no good. The heating element is definitely working - at least sometimes.

Does anyone have any clue what the problem could be?

Many thanks indeed