99 chevy Venture Over Heating HELP!!!?

[Philip D]

ok...here is what I have. I blew the engine in my wifes 99 chevy venture last year. The local mechanic replaced the engine with a used engine and I have had nothing but issues. He skipped town shortly after he replaced the engine and I was on my own to try and fix it. The problem has always been overheating. I first replaced the water pump then flushed the rad fluid and also replaced the thermostat. It was leaking antifreeze from the head so I did the blue devil stop leak think (I KNOW.... BAD MOVE).....now my heater core is blocked and I have no heat........what it does now is the temp stays normal for 5 mins....then it shoots up to just over the last white line but dosn't actually go into the red. if I accelerate it imediatley (like in 10 seconds) drops down to normal......then if I stop at a light it shoots right back up to just below the red.......could the temp gauge be wrong?.....can the temp change that quickly just from reving the engine??........I DID by a waranty with the engine (lifetime) and I DID locate the mechanic......should I take it back to him??......will he say its my fault because I used the engine stop leak??........what can I make him fix with a warenty????....please help!!!!!