ISX engine surging

[Roysbigtoys]

I have a 09' 386 Peterbilt, 485 ISX, 520k miles. About 6 months ago the mandate was removed ( a member on the forum programmed it and I did the mechanical part) and it has been running GREAT.
Today I first noticed that under acceleration it would rev freely up to 1500rpm.....then it was like a governor kicked in.....would only slowly gain speed. Gauges looked good, but the check engine light was on. It then started surging while I was holding the throttle still. The longer I drove, the less power I had.
I limped it back to my shop and will tackle it in the morning.
I finally have insite, but this will be the first time of me using by myself. What tests should I run? What else should I look for?