Sounds fuel system related. The carburetor may be flooding or rich. If you hold the accelerator part way down on a hot start, does it start easier?
The choke may be used first start in the morning, but should be wide open on a hot-start.
No spark when warm? You can try to chill various underhood ignition components one at a time until spark returns and the car starts.
Aerosol 'freeze' or other rapidly evaporating spray can be used. Avoid contact with skin or eyes.
Concentrate on the distributor pick up coil, inside the spark (lean burn) computer housing,etc. until you find the culprit.
Edit: If you do use spray inside the distributor, make sure that it is a non-flammable aerosol. An ohmmeter, if you have one would be safer to measure the pick up with.
Does it have a control module on the firewall? If so, it could be defective. You can have them tested at Autozone if you can find a COMPETENT counter man that knows what he's doing. This is what I'm referring to. Of course, this is without the lean burn engine. http://www.autozone....omString=search