Strange electrical problems, 1st gen

[Gardiner]

I have been experiencing random electrical problems lately and need some advice.

At highway speeds the truck sometime drops from 4th down to 3rd gear and then back up to 4th. The 'check engine light' flickers along with 'water in fuel' and the 'brake' light. The voltage guage will normally dip as well. It will also kick the truck out of cruise when this happens. The guage and lights have also flickered off/on at idle once before with a noticable change in engine pitch. This only happens every once in a while and immediately goes away.

In addition the preheated air start up system is currently not working. The thermistor, solenoids and the element have all checked out as good as well as all the associated wiring. These tests were done by Cummins techs, they said it was possible the ECM had a bad ground.

I figured that everything must be related to a bad ECM so I replaced that today with a Mopar unit. No luck, the problem did not go away with the start up element, and I'm assuming that the other problems still exist.

The TPS is less than 1 year old (replaced when the trans was rebuilt), all of the trailer hitch wiring including the controller have been replaced, the alternator has been recently rebuilt, the few accessories I have are running off of relays and the wiring all appears to be fine. The cruise, lights, signals, power locks, stereo etc all work.

I have just basic electrical & mechanical knowledge and don't want to keep spending the $$ having the techs try and trouble shoot the problem. It is just an intermittent problem so it is very difficult to diagnose.

Any suggestions?? It's going back to the shop this Friday. Feedback would be greatly appreciated