Stuck at 3,00 rpm

I have a 1979 300sd with a 1978 motor in it. It has new gas, oil, and air filters. About every other day you will be driving along about 75 mph when suddenly you lose power till you are at 3,000 rpm it will not go past 3,000 rpm if you take your foot off the gas for about 2 seconds it will accelerate past 3000 rpm. It also has done this once getting on the freeway in second gear going pretty slow, it has new fuel return lines they are not leaking. It has the old style primer pump I have ordered a new one. There does not appear to be any algae in the fuel filter. Any suggestions I could find nothing similar using the search feature.