You are spending a lot of $$$ , just a little at a time.
A good part of the time a coil with an internal problem will take out the PCM.
A valve spring for #8cyl could be with a problem. Because of the constant misfire problem the converter could be beyond use.
If there is no mechanical problem with #8 cyl , You might be ahead to replace a damaged PCM. There is a lengthy path to pursue because a misfire does so much damage and the info programmed into the PCM is always trying to prevent converter damage it will even disable an injector if it thinks converter damage is likely..

 

I just went through the same series of problems you described. I installed the same EXCEL coil packs and they are a good product, but there was a miss on #8. I re checked and found the problem. The problem was the harness plug to the cell pack. The two needles are easily pushed aside and will bend so when i saw the problem I corrected it and all is well now.
Best of luck to you.

 

Glad you found and solved the problem, thanks for the update!