I'm having some problems with a n14 cummins on 1994 Freightliner FLD. It seems to be very sluggish and shakes a little on a hard pull. Sometimes I have to downshift 3-4 gears on I-44 in Missouri. Its a 9 speed transmission. It uses a gallon of oil every 3000 miles because of leaks and/or burning it in the motor. I've been having some overheating problems at idle unless I turn on the fan. When I'm going down the road it doesn't overheat at all. Any ideas what a shop would say about these overheating and power problems? Any ideas on a course of action?