Could be cold-solder joints* in the main fuel relay.
*(fine cracks in the solder connections make the problem occur intermittently)

You could take the main fuel relay to a TV repair shop and have them re-solder all the connections in the main fuel relay.
_or_
replace the main fuel relay.

Also, check and clean all the grounds in your car (transmission, valve cover, body, thermostat housing, negative battery terminal).

Take Care,
ChrisS
PS, my 91 crx si had the same hard start problem after the car ran for 20-30 minutes and did a 5-15 minute stop on a warm day. Re-soldering the main fuel relay fixed the problem.