8,800 RPM is a bit low for the O.S. FS-91 Surpass engine. While we normallly use an APC 12 x 10 prop, we also get performance in the middle 9,000's. While your prop may not seem to be too much load, it may be that the airfoil and blade profile adds more load than the APC equivalent.

Try some different propellers. You want to have the engine running in the middle-9,000's or higher (you can go into the low-10,000's with no problems!) when it's leaned to just rich of peak RPM on the ground. If your engine can't do that, even with other props, then something's wrong.