Heat wont shut off ?

I have a problem with a Lennox system (at least 10 years old). It is a combo unit on the roof of my building. I could not get the heat to work so I called a HVAC guy. He came out and determined the main control board was bad. He replaced it with a new one and solved that problem. After he hooked it up the heat came on but would not shut off. He went into the building and messed with the thermostat and it shut off. After he left it worked fine for about a week then the same problem happened again the heat came on but would not shut off. The thermostat was pegged saying the inside temp was 100. they turned the thermostat all the way down and switched it to off still wouldn't shut down. I went up there and disconnected the RH and it shut down. I assumed the Thermostat was bad. I replaced it with an Electronic Thermostat and it worked fine for a few days then it stuck again, heat went up to 105.... Again I had to dismantle the thermostat to get it to shut down.... Any ideas on what to start checking ?
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