Go run the crap out of that thing. Really try to load it up hard. Cylinder pressure is what seats the rings to the cylinder walls. At idle, you have very little cylinder pressure. Under load, you have lots. Think about it this way. Under load, the burning fuel/expanding gases have to push very hard on the piston to make it go down since there is a lot of load on the engine. These gases get behind the piston rings and force them outward against the cylinder wall. So, the heavier the load on the engine, the harder the rings push on the wall. The rings are probably stuck, and no amount of idling is going to help them reseat. Get it out on the road and do a whole bunch of WOT pulls on it, uphill if possible. If that doesn't improve things, it's time for a rebuild.
