Engine misfire when warm

I am thinking along the same lines as Brad on this issue. Although you say you've had a recent ignition tune-up, this sounds like a classic case of a troublesome ignition. Usually, when the temp sensor for the fuel brain fails it will cause poor, hard starting as a result of a overly rich fuel mixture. This is caused because the resistor fails thus sending an incorrect input back to the brain indicating it needs additional fuel in volumes necessary for sub 0 temps. Once at temperature, the car resumes normal operation.

Try replacing both coils. They're inexpensive, and if nothing else, at least you'll have two new coils and you will have eliminated one likely culprit.

The only other way is if you have a shop close by that has a scope.
When a scope it hooked up to the ignition it shows what is happening when the spark fires. A good mechanic can tell ignition issues or lean fuel from a vacuum leak. should not be that much to hook a scope up to our old ignition systems as it is not a computer diagnostic.

Good luck and let us know what you decide or find.

Brad