6.5 erratic RPM's and Throttle
 

So I was wondering if I could get any advise on my truck. I have a 1994 Chevy k3500 that has the 6.5TD in it. I am having an issue with my RPM's and throttle being erratic. While at an idle RPM's are jumping between 1000 and 1200. While on cruse control RPM's are jumping between 2000 and 2500 RPM's at 55MPH. the throttle is VERY touchy and will not maintain constant speed, just wants to keep accelerating. Also at an idle or any type of acceleration I am blowing a TON of black smoke.
I'm getting codes 35,36, & 78
Now what I've done thus far!
Swapped PMD with a known good one
Swapped TPS sensor only with a spare
Took it to GM said it sounded like the IP because the truck itself was in time.
So I bought a tested good used one put it on last night still no change.
Swapped with another PMD that was going bad but would run the truck fine just shutdown after heated up, and still no change.
I even went as far as installing my 4" exhaust that I had bought a week prior to this starting, replaced the fuel tank because the old was corroded on the inside, also replaced the sending unit because my gauge did not work before hand. I replaced with clean fuel. No change...
Fuel filter and air filter are clean.
I'm completely lost I have no clue what this problem might be. Any input would be greatly appreciated!

P.S. at the beginning of the year I installed new marine injectors and glow plugs, placed the PMD on a heat sink and outside of engine compartment (inside of bumper). The lift pump was replaced 5 yrs ago and put on a switch to turn it on, and I can hear it running and fuel comes out of pitcock just unsure of psi

My 97 K3500 is doing the exact same thing and I have no idea why

Have you tried any trouble shooting yet. I'm thinking still a bad IP or bad fuel???

I've done everything you've done, it might be a bad IP I'm not 100% sure and I don't wanna waste that much money and it not be the problem

Try moving the PMD/heatsink combo from the bumper to near enough the IP harness to plug it in temporarily to eliminate the extension harness to see if a bad harness is the problem.

Also add a ground wire from the heatsink to the drivers side battery ground-shouldn't be needed but might help,won't hurt.

I've plugged the new pmd into the factory harness and no change, even tried it with a another brand new pmd I had still no change

Tried hooking pmd directly to oe harness no change. Sending pump back to exchange it and eliminate it as suspect, going to drain fuel put in 100% good fuel in it and see if that helps

Where was the PMD/IP harness ground wire fastened?

On the top of of the IP. Getting a new IP this weekend. Will have fuel drained to be sure I didn't not get contaminated fuel. Hopefully it will b fixed Saturday. Ill let ya know if it does. Also my pump was a 5288 so it interchanges with other pumps, though The used pump was a 5288 as well.

DS4831-5288 12558156 1997-98 LD & HD C/K, G, and P3 Truck & Van * DS4831-5459

This is the model of latest updates:
DS4831-5521 12561307 2000 LD & HD C/K, G, and P3 Truck & Van