So I just bought a 2000 Ford Expedition about 4 months ago it has around 160000 miles on it. Since buying it Ive had to replace 3 cylinder valves that were cracked and since the weather is getting cold ive learned that my heater doesnt work. My main concern though is my rpms are shifting weird it revs to higher rpms before shifting back down and every once in awhile the rpms will drop all of a sudden so i lose alot of power. I dont really have the money to go get it looked at was just hoping to get some suggestions on here on what the problem might be.