ISM MISFIRE @ 1500 RPM

[1096]

I'm having problems figuring this out. Misfire on #1 at 1500 RPM. Cummins ISM #35248744 320,000 miles. International 8600 (Thirteen Letter S___ Spreader)
I did a cut out test. Replaced #1 injector. Adjusted the valves. Rebuilt the Davco filter. new fuel lines. This is my other truck, not the one you programmed.
STILL MISSING ON #1 AT 1500 RPM. NO CODES before or after.
Next up: Fuel Pressure at pump. Clear hose at pump to check for bubbles. Inspect pickup tube in tank. Inspect Cooling plate. Ohm Injector harness. Swap ECMs with the other S___ Spreader, same ECM different program.

Got any suggestions? Or should I just keep on Parts Changing?

Injector did fix it, initially, lasted less than a day. before it started misfiring again. got injector new from Cummins.
WTF (what's the Foundation)