2007 International 8600 w/c-13

I am stuck with a 2007 International 8600 w/ c13. i have had issues with this truck from the word go. Since the beginning I get a check engine indicator when approching hills. It doesnt matter where my Rpm's are. The engine derates until acceleration is acheived or until I shut the unit off. I have taken this truck to Empire Caterpillar for all services and for a while repairs. The tech at Caterpillar advised me that the exhaust solinoid actuator (?) in 2 possisions needed to be replaced. This was the supposed cause of the indicator. It was not. Then it was exhaust back pressure. So I removed the cat back exhaust and replaced it with a staight through pipe. still indicator come on. My current mechanic says that he cant do anything, because Caterpillar doesnt share their diag tools. (?) I have always gotten around 4.5 mpg (shoot me) anythoughts?