Stumped by 97 Blazer

I have a 97 chevy blazer 4.3l 4x4 automatic that has had some issues running for a while now. Mostly through the process of elimination I have ruled out alot of things that I have replaced in search of the issue. Heres a list of what I can recall that has ben replaced.
fuel pump
fuel filter
injector spider (upgraded to get rid of the poppets)
crankshaft position sensor
distributor cap & rotor
ecu computer
coil & the part that mounts next to it, these came from a junk yard and were not new.

Recently I discovered there is spark from the coil, but holding an individual spark plug wire it wont get spark, but some did. And was not very consistant at all. I have replaced the camshaft position sensor as well, but again this was from the junkyard as well. I am pretty sure they can be tested at auto parts stores, but wanted to ask if there is anything else that I can check?

Also it does run sometimes, and when it runs it seems to run very good, but starts stumbling shortly after and then stalls and doesnt want to start up again.