along with that great advice I would rule out the thermostat and low voltage wiring issues.
1. pull the thermostat cover off of the wall. touch red to green wires together. does the blower motor come on at all??
does it stay on if you put a wire nut on them?
if so than you have a bad thermostat.
if not than yes I would say board and to check that you would need a meter. and like the good advice above. if you are getting 240v to the motor and the motor is not operating then it is bad. should be a run capacitor in there to check.
I was just thinking you wouldnt have the intermittent fan motor if a winding in the motor was bad it would just die.
I would check loose wires too. sometimes those wire plugs need a wiggle to tighten up. I would definately look there too.
make sure to pull the disconnect before working with wires, capacitors, motors too here is a quick
this is an outdoor disconnect so yours may be different at the air handler.