Help! Just bought truck 143K miles(from my son) Tranny is 45RFE i believe. replaced both AT filters sump and spin-on. added external oil cooler down line of rad. cooler. Acts like pulling load when starting sometimes. I started manual shifting from 1st. this helped but not satisfied with CEL on. replaced cam sensor and output speed sensor. After clearing codes and driving light would stay off until i would get up to highway speed then when i slowed down it would reset. I do a lot of county dirtroad driving on oilfield locations- rough country. The fuel mileage is poor also 10-12mpg whether ethonol or gas. Most of the time auto shifting does/sounds fine.
Is there any other thing to check? can anyone lend advice?