If I'd have to guess, I'd suspect the distributor pickup (Hall-Effect pickup) inside the distributor on the base plate. I've had a couple of those mysteriously go bad over the years and so have a lot of other people. With an intermittant failure the computer will not likely throw a code and because these often fail hot there is no really good way of testing without a scope connected when the problem occurs. Another possibility is a failing injector. We saw one of those fail a couple of months ago, according to another poster on this board. If you have a pick and pull salvage yard with those ,and can get them very cheap, it might help with the diagnoses without spending a lot of money on new parts.