My 99 Cummins has 90hp injectors, Edge chip and Fass fuels system. Within the last 1000 miles I snapped the input shaft so that got upgraded to the 1 3/8ths and also had the bearings in the transmission replaced. Just recently when the truck gets above 75mph on the speedometer it acts like you're choking its neck. Won't go much over 80. Runs rough as hell at this speed in fifth gear and about half as bad in fourth. It didn't do this before the tranny work. Something else I noticed was that before it started this, the automatic high idle would never engage. Since it started doing this it will high idle itself within 30 seconds of starting up. Not sure if it is related but the time coincided with one another.

 

Have a fuel gauge?

 

Both my fuel pressure/level gauges are disconnected.

 

So no fuel pressure gauge. Well I'd get that hooked up. Have you tried to get codes?

 

I have tried the key trick but have read that most '99s won't give them which mine fell under that. I am going to take in town now and see if I can get any codes from O'Reilly's scanner. I'll update on that shortly.