When the vent motor shuts off, check and see if you still have 120V supplying the motor. If so, then the motor is bad. If not, then there is a reason the power is being interrupted to the vent motor.
This could be a bad control board, short in the wiring system, an open limit, or a bad limit that is opening intermediately, a bad thermostat. Or, last but not least, a true problem that is causing the limit(s) to open.
The PSI switch is only closed after the venter motor is running long enough to create a negative vacuum, which in turns "closes" the PSI switch which then will start heat cycle if the limits are closed. If the vent is blocked, then the switch will never create a vacuum. Thus is a good thing. But i would say in your case, vent blockage is not an issue, even though don't take it for granted.
Does the circuit board have a green lite on it? if so, most time after system failure, it will flash 1-8 times repeating the same amount of flashes, informing of what failed.
See if it has so, post back.