I had coolant leaking thru the AC vents so i had the heater core replaced two weeks ago. Now i have problems that I never had before the mechanic touched it. I took it back the next day because the AC was blowing hot air, there was a squealing sound coming from the engine and the car was sputtering when I slowed to a stop. Mechanic said he fixed a stuck valve. Now the AC only blows cold air when Im not idling and even then its not as cold as before I had the work done. Last weekend I took it on the interstate and after about 35 miles it overheated. It seems to run fine if I dont use the AC but I also hear a loud bubbling from the coolant reservoir when I turn the car off. Again I didnt have any of these problems before the heater core was replaced and now the mechanic is telling me he will not look at it again without charging me more money. Can anyone tell me what could be going on?