Car is choking, losing ability to hold steady speed

With steady foot on accelerator, 55 mph at 2100 rpm, Diesel Engine. If I am supplying a heavy foot on pedal and accelerating the car runs fine, but after its shifted and I cruise with a steady supply of fuel at around 50-55 mph on a flat highway, the rpm needle will move from 2100 rpm to 2050 rpm back and forth, cough, go, cough, go, cough, go, with a mild loss/then gain of acceleration and then it keeps cycling like this a couple times a second. If I remove my foot and slow down its fine and the rpm's decrease, if I push the pedal it accelerates cleanly, rpm and speed increase steadily.

I've changed the oil, the oil filter, the fuel filter, and I've Sea Sprayed the engine without any change at all with this problem (although in general, the car runs better).