My 5.4 superduty was always a dog until I had it dynoed and had an excal II installed. The truck ran awesome for a few months then all of a sudden the tranny would slam hard from 1st to 2nd gear at light throttle. When I put the truck back to the stock tune the tranny would slip going from 1st to 2nd gear. The truck does take like 4 seconds to engage from park to drive.

I thought the slamming may be caused from the tuner somehow, but why does it slip in stock form. Now, after driving it for a week in stock form it is not slipping anymore, but still takes a while to engage into gear. Can anyone shed some light on this?

 

Perhaps the fluid is breaking down. First thing, if it's been a while replace the ATF. Tuners increase line pressure by spinning some pump faster. It's possible that you had too much pressure or the pump is beginning to fail.