I've got an IO360-A3B6D turning a Hartzell HC2YCR with a MT govenor. They all have about 680 hours on them. On take off the RPM has come down from 2700 to 2550 for the past 20 some odd hours. I tried to turn it up with the set screw but that didn't fix it. It made it worse. I did some tuch and goes the first take off it went to 2830 so reducing to climb out power I brought it back to 25 squared. When turning down wind I had over climbed to high and pushed the nose down to get back to altitude it over revved again. I took the govenor and prop to the prop shop and they said nothing was wrong with them. They didn't take them apart just tested them. I put them back on the engine and did a ground run. Now the RPM goes to 2720 on the ground with 29.6 inches of MAP. I brought it back and did a compression check. The lowest cylinder #2 is 77/80 at ambient temperature. The CHT on that cylinder is the lowest at 295 vs #4 at 318 being the highest. The EGT on that cylinder is the lowest at 1195 and the highest being 1325 on #4. The prop shop says the problem is in the engine. I don't see it. Any body have any ideas? Or should I move to a different prop shop?