Hotpoint TC71p condenser Dryer intermittent heating?

My condensor dryer heats intermittently. I have checked the three thermostats and the heater for continuity and all are ok. Occasionally, for a while we noticed that the reservoir light was staying on after it had been emptied, or came on when the reservoir was not full. It eventually goes out, but that hasn't occurred fora while now. I have cleaned the condensor and removed fluff from every visible part. Any suggestions why it should heat up only occasionally for a few minutes at a time then run cold would be appreciated. Sometimes it doesntbheatvup at all. Thank you.