Frank
The typical setup for a FOX 35 is to NOT put the tank on centerline, but to raise the tank 3/16 to 1/4 inch. Keep the tank as close to the motor as possible, Do Not use a OS NVA in it unless it is the OS 15 or 20, The OS 35S is much thicker and will kill off power on the FOX
I would switch fuels, and switch tanks, one after another, to see what affect it has, the prop, if balanced, should pose to problem.
And make sure your using 25% oil in the FOX or better, and I would recommend part of that synthetic
Let us know
Randy