My 91 fx liked a 13-6 APC prop for speed and I would run it at 12,500. I tried up to a 14-8 on it but wasen't what I was looking for. I also got rid of the OS #8 plug and put a Fox standard long glow plug in mine and made it run better.

Later on my 91 fx also had about the same problem you'res is having. I found out the bearings were going bad in mine so I had them replaced and it ran like a new engine again.

Hope the bearings are ok in yours.

good luck