I have a 15 yr old Lennox condensing unit 10ABC value series. It has been running perfect for all these years. Yesterday it stopped running. I did the normal circuit breaker checks and visual inspections and nothing appeared to be wrong. So I gave the unit a couple of taps on top and it ran. But only for about 15 minutes. I have done this a couple of times and I get the same results. Does this sound familiar to anyone? Any obvious things I may want to do or service/replace to get this unit operational? Any assistance would be appreciated. Thanks.