pdd81,
Electrical then !
Some heater switches have a thermal cutout in them. Sort of a Circuit breaker, which usually reset themselves after a cooloff period.
If your switch doesn not have that feature within it, it could be that there is another such item, somewhere in the circuit.
In any case, it appears that this is the problem, and I'm not saying that IT is the problem, but possibly a overcurrent from the Blower Motor, which is causing it to interrupt the circuit.
My best guess.
Have a good Day !
S.W.