Furnace diagnostic light indicates rollout open. Service tech came out and found that rollouts were not tripped. He seems baffled by the problem. The problem is inconsistent. The thermostat calls for heat. The furnace kicks the blower on without igniting burners. Cold air blows through vents. I will turn system off at thermostat and then turn it back on to reset. This will usually correct issue but sometimes it takes 2-3 off and on cycles to get heat blowing. Service tech now says he thinks it's the circuit board. I think he is guessing and am reluctant to spend the $170.00 on a hunch. I have already spent $90.00 on his service call and still have the same problem. Any ideas on this problem?