Im trying to figure out why my engine hesitate like it wants to turn off when im not driving.When i sit ata light or stop sign the car shakes like it wants to turn off. The rpm get real low and sometimes it do shut off When it drive it smooth out as regular. This happened right after i put my other engine in that i rebuilt. What could be the problem? Im thinking it have to do with the intake system. Should i take the intake system off the old motor and replace the one on the rebuilt motor. Any other ideas on what i shoud do?