Our 98 Freightliner with a N14 Cummins came in with unable to get over 1500 rpm periodically. Did some testing and found if I am below 1500 rpm and keep on throttle it drives normal but if you are above 1500 and completely let off throttle then get back on before it falls a little below 1500 there is nothing there and the engine will drop to around 1500 even with your foot flat on the wood. No codes coming up. Was serviced recently with all filters and replaced a leaking fuel shut off solenoid but the whole foot feed deal doesn't directly point to any of that??? I may go through and just play with all the connectors and clean them up first.