Intertherm FEH-017HAC won't shut off

[Jetherid]

Recently moved i to a house with this model furnace,looks to be pretty old but still hears up the place great. Issue im having is it that the furnace isn't always shutting off on its own, sometimes it works great, sometimes it just continues to pump heat into the house until I shut off the breaker. The Tstat is programmed to my desired settings, it has new batteries but it is placed near a door and shares a wall with the dryer. The evaporator coil is pretty dusty and needs to get cleaned up which I'm about to do, but not sure that really effects my issue here, either way its getting done though. There was no air filter in it either,so I've put one inside the top access panel but cant seem to fi d where it is 100%supposed to go for this model. Overall I'm just looking for some suggestions on what or if I can do anything myself before paying someone to do it.