Possibly a condenser or coil failing?  This could be showing up as the engine is heating up.  By checking the color of the spark from the coil wire that is removed from the distributor while turning the engine over with the key 'on', this can help to determine if the ignition system is 'weak'.  White or bright blue is a good strong spark.  Yellow or orange on the spark indicates a breaking down ignition that could be due to a variety of reasons.

Have you checked the voltage output on the charging system?  Also recheck all the engine grounds.  Performing a pressure and volume check on the fuel pump would also help to potentially isolate the problem.