1997 Ford F350 RPM surge

I have the 7.3 power stroke turbo diesel. I had the transmision rebuilt about 9 monthes ago and had a new starter installed about 2 monthes ago. Before the starter went the truck ran perfect. When I got the truck back from the starter repair, at 65 to 70 MPH on the highway it would normally run at about 2000 RPM but would surge to 2400 RPM about every 10 to 15 seconds and hold for just 2 to 3 seconds. When this was a symptem of the truck I also did not have cruise control. It did it for about 2 weeks. I returned to the repair shop that changed the starterand they said it was not due to the starter and could not repair it. Due to the fact that there are not alot of places that will work on my deisel, I tried to research the problem myself. Then after about 2 weeks it stopped surging and the cruise control worked. Now it is doing it again. I put alot of miles on daily but they are all highway. I really love this truck but cant seem to figure out what is the problem.