United technologies 1018 ignitor. Furnace comes on when it thermostat calls for it to sometimes. Other times it doesn't. If you move the thermostat above and below the actual temp, sometimes it will finally start. Other times you have to tap on the thermostat to get it to start.Have replaced thermostat so that is not the issue. Any suggestions as to what it could be?

Tapping to get it started would seem to indicate a loose or broken T'stat wire