Anyone? Any help would be very much appreciated at this point. Thanks!

 

I had pretty much the same symptom and it was tiny black rubber debris that had flaked off inside of small return line and was plugging hole on injector cap at hose barb. Blocked off the return, so IP wasnt "balanced" for lack of a better term. You just need to go over entire fuel system from tank foward and all the way back through the return before you condemn anything major
Even if you were to replace a bad IP, filter, Lift Pump, etc, the only way to get a 30 year old diesel to start easily and run right is to have a 100% perfect fuel system. I bet most out there have several leaks, debris, etc and owners are living with a truck that wont start after sitting a few weeks or dont run as well as they could