1995 GMC Sierra 1500 with 148k

So, I have gone through my 6.5 and replaced a lot of parts in hopes of fixing this hesitation issue

1. replaced vacuum pump (it was shot causing code 78)

2. replaced fuel pump

3. replaced fuel filter

4. replaced PMD with a brand new unit from Dipaco (used original harness and mounting location)

It starts up way smoother, and drives a little nicer, but it still hesitates under load. When I floor it, and let it wind up it dies and won't start again for a while.

What am I missing? does this mean I need to replace the IP?

That may be caused by Optical Sensor Harness? Suppose to stabilize the electric but when it is bad, it causes some hesitation/misbyte.

Is the the harness plugged into the top of my injection pump? How do I troubleshoot that?

I think so, I am not sure how to troubleshoot. May want to take it off see if it solves the problem.

Heath Diesel have a pic of it in their website since they sell it. Just go there and take a look at it.

Negative, the '95 requires it to be plugged in to work, ask me how I know ;-)

I know it has to be there but if it is bad, you have a problem.

Turns out it was the OPS causing trouble, the fuel pump wasn't pumping when the engine was at idle. Oil pressure shows okay on the dash, but the switch is burned up.

I wired the relay switch, and it works good now

Negative, the '95 requires it to be plugged in to work, ask me how I know ;-)
I disconnect mine couple years ago, and my '95 truck runs fine seems not all neaded the filter, but was installed anyway at factory.