I don't recall what pit you are using, a UDS?
It does appear the fan is way too big. I would get a smaller one but if you opt to throttle it down, do so on the inlet side of the fan, not the exit.
What tuning parameters are you using for the 3 PID parameters?
OK Joe Vertical Smoker, UDS, Komado Acorn, Weber Kettle Ranch, Weber Kettle, Stok Kettle