I am still waiting for the manual to show up before I go any further. Right now I still think the throttle linkage isn't quite correct and that's my problem. I will post if I find anything out. I do not experience any jumping of rpms. My rpms just seem to stay around 4500-4800 wot. I believe they should be about another 1000 higher.
Sounds like too much prop pitch . Mt DT-85 had a hard time planing with the 19 pitch that was on it when I bought it. Same symptoms as yours and about 25 mph tops. Now with a 15 pitch I get 5500 rpm and 34 mph and a great hole shot !!