Well it does sound like an AMM gone bad, I recently went through a very similar problem with on of my 86 245's and after a lot of head scratching and knuckle busting discovered that the dime store cap and rotor I had bought only lasted about a month (but hey i saved 5 bucks)replaced them again with higher quality parts and all seems well. 2 weeks now and not so much as a miss. If you decide to replace the amm i'd at least consider a used one unless you are 100% sure yours is bad.I picked up one at a local yard for $10 bucks and there is almost always a couple on ebay. the haynes manual has a description for checking them as well but i still can't figure out what they are talking about.
Good luck hope this helps
Bret
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rust free in west Texas