2002 Yamaha 115 4S top end RPM

[George Allen]

2002 Yamaha 115 4 stroke ran rough at low RPM (below 1500). Dealer replaced fuel filers, cleaned fuel tank on side of engine and cleaned fuel rail. Engine ran good a low rpm but decellerated when pushed above 5100 RPM. Dealer indicated thermostat was sticking and replaced. Now it is about 85% better than it was and will turn up to 5500 RPM but with the last 1 1/2" of throw on the throttle, the RPM drops 200 RPM. (Didn't do it before the throttle body maintenance.) Fuel is fresh non E-10 BP gas.
The water discharged from the engine feels luke warm. Don't know what the t'stat is susposed to maintain it at. Help.