Hi all, i have a 99 Ti with a 4.5 auto.

Last week i picked up my truck from the panel beaters and as i took off 1st to 2nd gear changed with a kick like it had a shift kit and when i took it on the highway the converter wouldn't go into lockup while i was accelerating, BUT if i set the cruse control at any speed over 80 the converter locks up!
I think it could have a suss TPS because i couldn't get a proper reading off it when i was trying to re adjust it for harsh gear change but that wont explain why it only locks up while the car is driving on cruse control.
Has any one else come across this??
Any help would be much appreciated. Cheers!!