X15 surging under load

[FueledUp]

Trying to help my neighbor out this morning and I’m at a total loss. CM2350 X15 demandated by a less than ideal individual about 6 months ago. The problems I’m going to state started about 2 months ago and can only be replicated under load. Bobtail or empty trailer you’d never know the truck has a problem.

So the truck UNDER LOAD feels like you’re riding a bull at the rodeo. The throttle position sensor is ALL OVER THE PLACE! 100% to 0% to 75% to 100% to 33% etc… When empty, no such issue. He doesn’t know which codes it threw other than a cylinder unbalance.
Peterbilt replaced the following:
- Fuel Filters
-Throttle position sensor with lead
- Fuel Pressure relief valve
- Fuel pump actuator
- ECM

After spending thousands of dollars and no resolution, he pulled the truck and it has sat since. He brought it over to me last night. Lift pump pressure is 25psi. High pressure fuel pump puts out 150ml @ 140rpm in 30 seconds (although the fuel coming out of the rail looks to have a LOT of air in it. Fuel from fuel filter has no air in it). Turbo passes on insite. Injectors pass performance test EXCEPT on the 3rd test on each injector). I only had about an hour to commit to it last night. Going to check throttle valve this morning and do an injector leak test. Just not sure where else to look. Help would be appreciated! I’m not a shop. Just a fellow trucker trying to help another trucker out. Thanks!