When I drive my 1997 Subaru Legacy Outback wagon and the cars temperature gets up to operating temp, if I come back to my car and the guage is more than 1/3 of oper. temp. the car will not start. I have to wait for the temp to go below about 2/3 and the car will fire right up. I have found no answers on the internet or from any mechanic I have asked. We have tried switching the Mass Airflow Sensor to no avail. Maybe a relay?
Maybe a temperature sensor?
Have you tried a ‘flood clear’ procedure when this occurs? The usual procedure is to hold the gas pedal to the floor while cranking and easing up as the engine catches but check with your owner’s manual.
As mcparadise suggested, have the mechanic check the calibration of the Coolant Temperature Sensor especially when hot. If it is telling the computer that the engine is either too cold or too hot when the engine is to restart the amount of fuel injected will be incorrect.
Hope that helps.