Why is my 1995 Bmw 325is overheating?

When I would drive on the streets and accelerate on the gas, the temp, of the car would go up and when I would stop it would go back to the normal temp. But sometimes It would be the opposite the temp. would go up when I stopped and it go back to normal when I pushed the pedal. Recently, my car overheated while I was parked and the cooling expansion tank blew up. I replaced it and the car is still overheating and not going back to normal temp. Please Help me???? Id appreciate it:)