You should be able to unplug the blower leads from the circuit board and rig 120 volts to them. Note, you should have at least 3 wires, a white neutral, a high, and low speed lead. There may even be more allowing different speeds. With the neutral connected to a neutral, connect a hot wire to the low speed lead. The blower should start up. Since it is intermittent, try it several times from a standstill. There may be a schematic to help identify the low speed lead. Or you could just check all the leads.
Chances are, as the others said, it is the relay, and you will have to change the board. It would be disgusting to spend hundreds on a new board, and then find out the problem was the blower motor or the flame detector. If the gas goes off again a few seconds after it comes on, likely it is the flame detector. If it stays on a few minutes, likely it is the relay.