1977 AC Blower Motor Problem

[GLASEM]

My AC blower motor seems it cannot decide when to run.

Will run for half hour then quits. Next time I run the car it works no problem.

The next time it won't run.

Is the motor going??? Control switch??? Relay

Any idea of cost of new??? Besides Pelican any local type auto parts store carry??

Would consider a good reasonably price used also