Your problem sounds like a lean midrange. If so try running the main needle richer than normal and see it helps the transition. I don't think the Fox runs as well on a Perry as the MK series Fox carb. At least my Eagle I prefers the MK over the Perry. If you have one around see if it cures the problem. The carb may be smaller than stock, so if this works, maybe the new Fox twin needle carb would fix it.

A lot of Fox's have aggressive timing, but they are not the only ones. I have an older Super Tigre that is very trickey to tune for the same reason. A carb that has full adjustments works well.