So my truck starts bucking at about 40mph, the rpms start going up and down repeatedly. If i mash the throttle she goes just fine, if im cruising on a straight away at a constant speed of 30 - 40 mph . i get the idle jumping / bucking. I had this problem once before, but it went away after my injection and lift pump were changed. So im just looking for any possible answer, could this be a sensor of some sort?? My truck is a 2002 cummins diesel 24v with 340k, automatic trans. Thanks for any and all help!