I have a friend that has a 96 3500 6.5. The motor will start and just a few seconds later the RPM will jump around from 800 to 1200 at idle them even out then back to moving around. It stalls at times and may not want to start right back up and after trying a few times more on cranking it just starts back up. It doesn’t have a lot of black smoke just the norm. When you drive the truck down the road and the motor starts running funny the Trans will come out of lock up to but not all the time.

Start with the basics.
1. Check to see if the lift pump is working. Open petcock on thermostat housing. Fuel should come out. If not, engine will die in less than 30 seconds.
2. Make sure that fuel filter is clean.
3. Check back and post results from above.

Only two things will make it seem that underpowered. A serious lack of fuel supply pressure/inline restriction or limpmode.

The erratic idle and stalling seems to indicate fuel supply pressure.

Well just checked fuel pressure and all is well. I performed basic diag with snap on scaner and found codes p0118 coolant temp and p0236? What is p0236? I was watching the coolant temp sensor on the scaner going from -32 to + 65 degrees and timing was following from 10.0 to 14.7 degrees. I unplugged the temp sensor the one on top of the motor and the idle rpm went to 950 and leveled out. Truck starts fine now that the sensor is unplugged. What is going to be the out come if we leave that sensor unplugged? Will the pcm default to othere temp sensor for the trans to shift right?

Sounds like a bad sensor, its probably preaty cheap to replace it. by unplugging it the ECM goes into a closed loop mode rather than reading off the sensor. If it was me I would just replace it.

I get the SES light when my ECT sensor is unplugged and 950 rpm idle, lots of black smoke due to extra fuel. Having a working sensor is the only sensable solution but make sure the problem is not in the wiring harness.