2006 DAF cf85 will idle then go into a limp mode?

When driving the truck idles then loses power and gears down like it goes limp with no power. It takes a few to get exelerater back to be able to drive it. Wiring harness replaced computer module replace. Mechanic says no faults no warnings other that air suspension that goes away with a trailer attached. Drove it out within an hour problem back twice. Drove it back 3 times. Frustrated and losing thousands by the day. Already off road for a month any ideas