N14 Celect Plus No Power, Won't Rev Past 1100 RPM, No CEL

Hey all,
Driver recently experienced a random severe loss of power driving on flat ground. I'm thinking this is a fueling issue as the CEL didn't come on. I'm having the driver pull the fuel filter and replace it as a quick check, in the last maintenance period we did see some gelling and treated with biocide. The driver is also reporting a whistling coming from the turbo side of the engine, I repaired a boost leak yesterday thinking that was the issue but the driver reports that didn't fix it. Any ideas on other issues with the same symptoms?

 

In neutral it'll rev to the limited RPM of 1500, its only in gear that it struggles. Driver said that at 1100 RPM the engine jerks like its shutting off and then recovering on repeat, but the reduced power is continuous. Once he gets back I'll be capping the pressure side tubing from the turbo outlet to the intake and checking for leaks. I couldn't hear much yesterday since I did it roadside with the engine running and the drivers coffe mug jammed into the intake manifold coupler. How does one check the fuel lines for holes, pressurize the saddletanks and look for wet spots?

 

I checked for freeplay yesterday as that was one of my biggest concerns. There was no turbine thrust and a barely perceptible amount side to side, no housing contact though. I'm sure the couplers are at fault here as they are in pretty rough shape.I will be putting in a boost gauge to see the boost being made.

 

Update on the truck, removing the fuel filter was darn near impossible. When I finally got it broken loose, air rushed in. I'm thinking the saddle vents are clogged but I haven't been able to recreate it. Truck runs like a top now though.