05 9900i

i owned one of those trucks,spent a good chunck of money trying to get it to run like i want it to till i decided to get rid of it.are you getting any codes?from my experience thats a bad engine speed sensor.2004 and 2005 cummins isx are the worst cause they are the trial engines for the first round of emmissions regulations.good luck

 

Sounds like a split second loss of ignition power. Ignition power to the key switch is normally from breaker F7-A, 25 amp. Make sure the socket terminals the breaker makes contact with are still good and tight. After that breaker the power goes through the key switch and direct to the ECM and ignition relays. The main power feed for the cab usually goes from the positive post on the starter, up to the firewall on the right hand side to a 150 amp mega fuse, then inside to the main copper bus bar at the breaker panel. Make sure all that is good and tight too. Remove your kick panels under the key switch. Start and run the truck and start wiggling everything you can get your hands on. Be careful around the engine, wiggle the engine harnesses where ever you see them. Hopefully you can fine your bad spot. Try a new key switch if you find nothing else.

 

on top of what HeavyD said,look at plastic wire loom going across engine valve cover.if it is touching cover,look to see if it has rubbed through the loom and wore the insulation on wiring.if it has look for broken pink wire (key power to e.c.m.good luck.