I saw your earlier posting. Wanted to let you know I'm having the same problem with my Z28 130k miles.
Missing at low RPM.
I've replaced the OPTISPARK, plugs, wires, coil, ECM and CAT. All for another problem, the seal went bad in the distributor and filled it with oil, frying the whole system. The CAT for a failed emissions test. The missing thing just started.
Please let me know if you find anything. I'm stumped as well. I'm thinking a worn out timing chain or vacuum leak.
Glad to see I'm not the only one. Maybe together we can figure it out. At least I will save some money not replacing good parts with good parts.
Please let me know what/if you find anything. I'll do the same.
Good luck!!
Steve