I've got a 1999 Chevy 3500 with a 454. I have rebuilt the motor and transmission and replaced everything from bumper to bumper in the engine, fuel system, all new sensors, and put in a custom-tuned computer. The truck runs great empty, but when I hook a trailer to it there is a bad miss and it backfires out the intake and exhaust. I've had it to several different mechanics and it has been put on several different computers and no codes come up.
Does anyone have any suggestions?

 

I'd question the custom tuned computer

 

it was worse with the factory one