My new Knight Hauler/Freightliner

[540-MAN]

I'm having a problem with my truck now so I hope somebody can help me with it.

I gave it a test run the other day and everytime I put it in third gear with trailer on and the stick is flat out on the radio it shuts the truck down. Then when I take the trailer off it seems to run fine with out any problems.

I have checked all the wiring to make sure everything is connected and even fully charged the batteries but still no change so it has got me stumped of what is wrong with it.

Has anybody got any ideas of what it might be?