Well, to eliminate this as a problem, try cleaning out the fuel tank cap vent. The cap vent being stopped up (or partly so) can cause symptoms like this, especially if you begin mowing with the tank completely full. A good way to check this would be to run the tractor long enough for it to stall and die, then remove and retighten the tank cap. If the cap vent is clogged, loosening the cap will break the vacuum that develops when fuel is burned. Because you notice a difference between full throttle operation and 3/4 throttle operation, I would say your cap is not completely clogged, but very near clogged. Anyhow, this is cheap and easy to try.

Thanks for the tip Mownie. I will give it a try.