If it were the exhaust back pressure I would think that it would be more of a
complete lack of power. But if it goes stupid, it could be...
I would start with boost, what boost pressure do it have? If you dont have gauges
then it is just a guess, so time to get at least a temporary gauge. Did this happen all at
once or over time? As stated above, check the intercooler and turbo boots.
Also, check for rubbing on the intercooler tubes. There can be holes where rubbing
happens. If you havent already, you need to
eliminate the rubbing from the MAP sensor hose, and the battery cables. I have seen
them wear clear through the pipes and cause the same symptoms. Worst problems
are usually on the passenger side (MAP tube and battery cables rub on every stock
7.3 I have seen.