No hot air with filter in place

Hi, my forced air gas heater recently developed a problem. When I have a filter in, the heat will unpredictably stop heating and blow cold air instead. I read about this being caused by a dirty filter and when I took the filter out and reset it worked fine. However when I put a new filter in I have the same problem. I tried the least resistant filter, MERV 4 or 6, and still had the problem. With the filter now out it hasn't happened in over a month. I'd like opinions on whats wrong before I call a repair guy.