Intertherm furnace. We have an intertherm furnace that won't complete a heating cycle. I have to

Question

We have an intertherm furnace that won't complete a heating cycle. I have to hit the reset button until eventually it will or won't. In the winter months we have to have it serviced pretty regularly (we live in Alaska). The service guys say that it gets sooty and they have to clean the eye (something like that). They can't come up with a reason for this other than the combustion air is not clean. It pulls air from inside the home and we don't smoke so I don't think we are really getting to the root of the problem.