96 gmc 1500 4.3 liter skipping

There is many reasons and items to check/test. Since the actual items the computer is pointing to I believe there is a low power situation, Bad/loose/corroded/ wiring or connectors. Go threw every ground to frame/block/battery/ECM to ensure they all are good. There are many all over. Look for wiring issues, scrapped/bent/frayed/dirty/corroded/rusted ground connections/ Also the same for the charging/starting/positive side of wiring and look for the same. Also use a multimeter to test the ohms & resistance of these wires, Somewhere your loosing voltage in the system, I do not think of it as a parasitic draw that does so while off but does so while on, A wire or even wires are not able to carry the current. The repair manual even shows the Starter itself as a possible cause where the power is lost so even if its working it could be drawing power out of the system. I would check for parasitic draw (test with key on and key off and test battery terminal and connections to starter) just in-case it shows or points you to the spot or area. The alternator may not be able to put out full power (14+volts). Since the repair manual points to the starter motor I would check connections to starter and terminals wires & terminals and ground for battery and then if needed test the starter itself at a parts store for open circuit inside. Test all the harness to and from the cam and MAF sensors and at the ECM from harness & ECM ground and fuse panels/blocks. Also you need to locate any air leaks from and around MAF to engine and correct even if its the intakes gaskets, there should be no air going into engine without first going threw the MAF. Check the PCV. Look for any vacuum leaks and fix,, cracks/bends/weak rubber in hoses ect. Any air that enters the engine w/o passing through the MAF is un-metered. Test and check EGR. Catalytic converter plugged up? If you can't get exhaust out you cant get air in. TPS and MAP sensors can change readings fine, but MAF will not, ECM catches it and throws MAF code for plugged cat. I have even seen exhaust with rusted loose pieces of muffler in pipes that flip up bog motor and back down when engine bogs then back up when engine rpm's raise back then do it all over again which in this case would set a MAF code like its a cat fault. So check exhaust and for bends too.

Don't have cats have o2 sensors in muffler pipe tho cut hole in pipe and then weld couplins to screw them in and I have changed ground strap from motor to frame and frame to cab cause one look like it got burn by mffler or something

I have one like that too, no cat and o2 with welded in adaptor , 1992 4.3l it started now to suck air in from the intake and is idling high (bout 1900rpm), I plan to put intake gaskets on soon.

check the distributor gear - mine was wore and misfired like crazy