I just had a Jasper rebuilt motor put in my truck and am having an issue. Noticed shortly afterwards that when I have my trailer hooked up, that when I back into my drive (has a slight incline), that the motor surges at idle or little above as I back in. It is like you have your foot on the gas and then your foot slips off then you give it gas again. When I take off in forward it does it but not as bad. It is the 4.2 engine. As soon as you get up to a little speed it runs great. They cleaned the idle control valve but it still does it. Any ideas?