For almost a year I have been struggling with a recurring problem that
appears to be triggered by sustained driving on the highway. It has come up
three times now and I thought I had it fixed after the second episode so I
don't know what to try next.

It's a 99 Accord LX 4 door 4 cyl 5-speed, 184,500 miles (lots of highway
driving).

The problem is after the engine hits operating temperature, it cannot idle
at the bottom end -- when you slow down it just drops off to zero RPMs and
dies unless you feed it gas with the clutch down. It first happened during
an all-day drive last summer. I had just changed the oil, filter and air
filter for the trip. To try to fix it, I replaced the spark plugs and the
PCV, ran some injector cleaner through, and replaced the coolant and the
ignition coil. That seemed to help a little and the car stopped dying so
much but still ran uncertainly at low RPMs. Then I took another long drive
(this time a warm day in December) and the same problem came back. This
time I switched out the ignition wires, distributor cap and rotor. Lo and
behold the car seemed almost back to normal. I have driven it up to 50
miles at a time without incident. But this weekend my son drove over an hour
on the highway (warm abient temps, too) and sure enough, it started dying
again.

There are no trouble codes being activated. I'm looking for a good idea on
what to try next and this group is a great help. Thanks in advance for your
suggestions.