02 cummins.6 speed.. have replaced apps twice... have hypertech module and 5 in,. exhaust.. runs good without trailer...new fuel filter & replacesd electric pump on side of block. engine surges or cuts out under a load... any suggestions appreciated 168,000 miles

Do you have gauges? They help answer most question. Did you check for any codes. Turn key on, off,on, off, on and leave it in the on position. Try that. You might have vp44 troubles.

check codes but sounds like a low boost/map sensor problem possibly if it only happens under a load. could be other problems but first check for codes