1996 Dodge Caravan Cause unknown

Electrical problem
1996 Dodge Caravan 6 cyl Front Wheel Drive Automatic 238kms miles

Have vehicle stated. Runs fine. When warm and shut of for a brief period (erraticly) may re-fire and only run for a few seconds and quit like the key had been turned off. Will not re-fire for use until sufficently cooled. Often for houres later. Sometimes does and sometimes not. Had to Mechanic, and a fair one, claims cannot find problem unless it behaves in this manner while he has it. Very very frustrating. Help.