I have a 1998 12V. Just came back from a 2500 mile run. I had a 6 X 14 dual axle trailer with 3 Dressers (full size harleys) luggage and 4 people. My problem is, I could not get over 70 mph and as soon as I hit any hill, she really slowed down and the passing gear would kick on but the truck would actually slow down to around 45 mph from 65 mph, like the brakes were on. It would also throw smoke. It would down shift and sounded like it was trying but just wouldn't pick up any speed. I have no idea but it certainly should have pulled better then it did.
Help, I'm baffled. It has 158,000 miles. Runs fine with nothing hooked on the back.
I checked the clamps and all seems ok. Just for giggles I ran the rpms up in park (floored it). It winds up to about 2000 rpms then studders, smokes and clears up and winds back up. I stopped at around 2700 rpm.

 

As long as there aren't any boost leaks my thought is a leaking AFC line or the AFC diaphragm is cracked.