Are you using the same tc over and over? if one tranny failed then the crap could be in your tc and you are just transfering the junk to your new tranny.

Any the stock tc is barely able to handle the power of the stock truck.

 

If it's the same issue with all 3 trannys, then i'd start with what was suggested earlier, the TPS wiring. It's obviously an issue with the truck not the trannys.

 

Jasper trannies are not bad, i have one and a few memebers here can vouch for me when i say i use the truck pretty hard, and KNOCK ON WOOD, it's still operating just fine...