Ruud heater will not light consistently

Question

i have a ruud heater and at times the ignitor doesnt come on, which means the gas doesnt come on and the furnace doesnt light. when the thermostat turns the blower fan on it continues to run and doesnt shut off. this problem was intermittent but seems to be happening all the time now. whats really strange about it is that if i go to the unit and turn off the power switch at the unit and turn it back on, the ignitor lights and the furance kicks on and everything is ok until the next time.