Engine still not running right..................

After running for 5 min or so it idles great and throttle response is good but after it has ran for 15-20 min going up and down the road, then it runs like crap but runs real nice at idle. It hesitaes a bit if you rev it up when good and warm. Going down the road at half throttle, it seems to miss a bit or rev up and down a bit. Only does this when it warmed up. Sems to run great when not warmed up. Temp gauge is on the line between cold and run and never goes higher. Oil pressure is really good when fired up but drops off to almost nothing when warm. It goes up if you rev it up but drops off when you throttle down.

Anyways, put new bearings in the governor and have a 2-1/4 X 3/4 spring installed.

I'm Lost!