inconsistent running

[bendor]

So here is my dilemma. i purchased a 2008 kenworth t 800 tractor with a isx 15 cm 871 485 horse with about 800.000 miles on it about this time last year from a out of the area kenworth dealer. I drove it back appr. 400 miles then it sat most of the winter because its a construction truck. When I purchased it they told me that they had just put a new turbo and actuator and dpf on it and i believe a new dosing valve. It looks like they did they had shop repair invoices. Upon trying to put this truck to work I have had nothing but issues. What it does is this. It could be day to day, hour to hour or even from shift to shift and seems to be getting worse as well it seems to me to be regening way way to often a couple of time a week it has less than 11000 miles on it since purchased. It will run up to 2000 - 2100 rpm and run good seems to have 485 horse but then it will sometimes only run up to 1700-1800 rpm and run or pull like it is derating to around 400 horse as well sometimes you will be accelerating and be about 1900-2000 just ready to shift when it suddenly acts like you shut the key off until it slows down to about 1600 then it will actually start to run like its derated again if you keep the pedal depressed. So far the local kenworth dealer has taken it in and each time said there you go its fixed. So far first time they claimed low coolant and replaced the coolant cap and cleaned the battery connections at that time they indicated that the turbo speed was a little low but should be good.2nd said it was charge air cooler so replaced that (nope). 3rd time finally quit so they replaced the ecm (nope). 4th time said throttle calibration it was fixed (NOPE). 5th time running down the road I believe the rebuilt turbo failed made a horrible noise like it was chewing itself apart then quit running. they agreed turbo failing. this has been going on for 2-3 months. At this time they will not warranty the turbo because it is out of warranty date by a couple of months. I am at my wits end this has to stop I guess its up to me now where do I start and look? I would appreciate any help you could give me! I am used to the old big cams and n 14 but know nothing about these isx's . Any and all help would be appreciated thanks again