I have a Carrier model 58SSC that is about 15 yrs old. About 3-4 months ago, the unit shut off. I replaced the filter, reset the power and all was fine. Then about 2 months ago, it quit again, since then it has been happening with increasing frequency - about every day or two. All I do is turn the power off then on and the furnace works but when the system is off, some of the times, it does not kick back on until I turn the power off then on. What may be the issue??