2000 Chevy Lumina heater not blowing hot air and car temp r

I just bought this car from a used car dealer ship.. And 3 days later my service engine soon light came on. I thought it was probably am emmisions problem and didnt think much else of it until. I was driving home that night and my heater would not blow hot air only cold.. My temp guage would rise and fall but never reaching the hot side only in the mid range..But the next day i went to drive it and after a few miles it started to climb to the hot range..I checked the coolent in the reserve and in the radiator and they were both fine.. I am wondering if it is a thermastat or maybe my heating coil.. I am baffled as to the engine running hot..Could it be a blown head gasket.. even thought there is no water in my oil..