Were you able to get hold of an OBD-1 reader or did you figure out the paperclip trick?
Schwind's post #3 on the
'A Little Help Here' thread explains the procedure for using a paperclip to pull the codes. Here's the full link if the hyper doesn't work.
https://www.thetruckstop.us/forum/threads/a-little-help-here.2657/
With all the issues, sucker's gotta be pushing codes, even without a check engine light.
Air Cleaner and Turbo aren't the cause for it running up to 4-5000 before suddenly dropping to bounce between 800 & 1500 and the term 'injector locked up' is new to me.
If it is chugging, not hitting on all eight and feeling like its going into death throws etc. I would be leaning heavily to the IP AFTER diagnosing the fuel system from tank forward. Bad IP may not be reliably commanding fuel to all cylinders as desired. This could create a 'dead hole' or bad injector symptom.
A mechanical pressure gauge attached at the T-valve located at the front of the motor by the T-Stat will give you some idea of the state of the fuel system. At least with 4-5lbs at idle, you'll know not to toss money that way. Picture credit to TurbineDoc.
More part locations help can be found in the 6.2 6.5 Technical Library thread,
Part Locations.
https://www.thetruckstop.us/forum/threads/part-locations.9494/
To go over your fuel system from Tank to IP, TurbinDoc's old thread is a good time killer.
https://www.thetruckstop.us/forum/threads/lift-pumps-ops-operation-and-troubleshooting.6872/
However, if the IP has gone south, no amount of new PMDs, APPs, OPSs, fuel filters, lift pumps, tank socks, air cleaners or turbo's is going to solve it.
If you do decide to go in deep and need a set of manuals, I have a 4 volume GM set you could use. Just PM me your address.