turbo lag and variation

[Milehigh]

I have a 2008 cummins ISX that I am experiencing difficulties with. The turbo has quite a lag or hesitation when you step on the accelerator. It makes shifting smoothly almost impossible. The turbo boost is also very unpredictable. Some days I will have 41 pounds of boost and other days only 32-35. I have even noticed on some long pulls that it will fluctuate 5-7 pounds of boost back and forth while on a dead pull. Another odd problem is that often on a hard pull the turbo boost gauge will drop to zero and a check engine will flash over the screen. It will flash for a bit and then hold steady. Sometimes it only holds steady for a few seconds and then all goes back to normal. Other times it will hold the check engine code for the length of the pull or until I back off the throttle. One mechanic wondered if the band for the variable turbo was sticking. The turbo is supposedly only a year or two old. I have had the truck for 4 months. It was mandate'd before I bought it by {*** Name removed: PM for the info ***} in Colorado. It was stock a 485 with 1850 torque. I talked to the man that sent the ECM in and he says it was flashed to around 600 hp. The motor has 765000 miles on it and has not been overhauled yet. Am looking for help.