When the thermostat calls for heat the elements heat but the blower does not come on even with the switch in the run position . This happens randomly if you turn the switch from auto to run several times it will eventually turn the blower on . What part needs to be replaced ?

What part needs to be replaced
I don't know, but I would be looking at the fan relay. Sounds like it is failing and by switching back and forth between fan and auto you are intermittently getting it to work .