I found the problem on my son's Omni ( yes, MANY years ago!). Cold start, fine, but as it warmed up, it would miss. He drove from Cinnci OH to Richmond, VA for a gig, all in short spurts. Every shop would replace something different, and send him on his way.
I reached down to the distributor while it was running, and got shocked. Found that the module on the side of the distributor was bad. Sometimes mechanics don't take the time to find the underlying problem.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags