I've got a 98 12V that has developed a couple problems lately.
1.it has become very slow to turn over.Sounds like starting with weak batteries but they both checked good.I have cleaned all the terminals and it hits a little better but still not right.Thinking maybe the factory ends on the cables are corroded inside.

2.Truck will intermittenly die while stopped and in gear.Will run all day in park.It doesn't do it all the time.Will crank right back up.It just shut off,no stutter,no warning,like you killed the ignition.Have been throwing it in neutral when comming to a light because of the weak cranking. Truck is an auto and the idle is set at around 775-800 in gear.It has 370 injectors and the starwheel adjusted slightly forward and the stock plate full forward.Going back to a much smaller injector next weekend and putting the plate and wheel back stock because I'm trying to sell it,but I neeed to get these 2 bugs worked out.

 

You need to find someone / shop that has a load tester , to see what the starter is pulling or not in amps , you did not mention a load tester , so I would not count out the batteries till I knew what checked out meant .
If it is the starter , you should get the larger solenoid contacts , at least to help the starter you have [ there are other parts to a rebuild so these are not meant to be everything for a rebuild , just an upgraded part ]

http://www.fostertruck.com/dodge/default.htm

The shutting of may be a number of things , throttle linkage / adjustment .
The fuel shutoff solenoid , or electrical to it , like relay , then also ing sw.
This you start with a test light to make sure that power is getting to and or through all of the fuel / ing systems .
Get a factory manual to have good wiring diagrams to chase with the test lite .

 

Like John said. Might want to check these I posted:

https://www.dieseltruckresource.com/...arter+solenoid

 

does ur truck try to ease forward when ur stopped in gear with ur foot on the brake and then die if it is then ur tc is locking up and killin your truck mine use to the same i had to bump my idle up a little bit and it helped