I bought this 94 ZJ Jeep Grand Cherokee 146,xxx miles back in February from a retired officer Vietnam vet that took phenomenal care of the vehicle (original owner, had all the service records). Vehicle runs great a little hiccup from here and there nothing major but it’s an older vehicle so it’s expected. It’s not my daily driver, since I’ve bought it I’ve put less than 2,000 miles on it. The problem that has come about now since it’s winter has me very perplexed. After driving for about 5ish minutes with the heater on and water temp gauge at roughly 120f the heater will only blow lukewarm air. However the water temp gauge will rise all the way up somewhat rapidly then will drop back down to what it should be normally reading only then will the heater kick on. I have only drove it about 10 times in the past few weeks and I want to say 8 of those times this has happened. I have spoke with friends and checked out every forum I could find, have been looking at possible heater core, thermostat, temperature sending unit, maybe temp sensor? Any ideas/advice would be greatly appreciated!!