1995 ram 1500 surging

I have a 1995 dodge ram 1500 with 200,000 miles and have put a new engine and rebuilt the transmision. Now I am having a problem with the truck surging ant stop lights while I am in gear, when I put it in park it stops surging. It runs smooth any other time can any one tell me what to check. Oh the engine is a 5.2 liter or 318.

Vacuum leak maybe? With the new motor, you may have missed a connection.

There is a possibility that it is your TPS (throttle position sensor).