It does sound like a rich idle is loading up the engine from extended low speed running. When you adjust the idle mixture do NOT crank it shut and start over like you did the high speed needle. The idle mix is VERY sensitive and should be done in 1/8 turn increments to avoid going too far. OS routinely sets the idle mix rich on new engines to avoid lean runs that will damage the engine.

Stop the engine for tuning to keep your hand out of the prop.