If its slipping, then the servo or clutches are not holding. 3 things could be happening. Either the frictions are no longer bonded with friction (worn out) or the seals that are on the servo/clutch pack are bad and are not able to hold the pressure to the actuators, or the pump is shot and is not generating enough line pressure to do its job (this could be lack of fluid (not likely as you checked it) or a stuffed filter that COULD cause something like this. Im sure the deep pan came with an extension and a filter, may not be an issue depending on what you did in there.
