Why my engine stalls when it runs at idle?

I have a Renault Megane Classic cc1.4 mod 1999
The engine stalls for no reason. Usually when at idle, stop lights etc. But also when slowing down to stop. Altgough many times it runs normally at idle speed sometimes unexpetlendly it dies. The problem seems to occur more frequently when the engine is hot (and the weather tepmerature is high) while runs idle and the A/C is ON. The ecu seems to get very hot and Its hard to restart the engine for about 5 minutes. When the key is on (No ignition) the fuel pump does not operate. If I disconnect the ecu plug and reconnect it to the ecu the pump operates and the engine starts and operates normally. This happens for a few days in a row then quits for awhile but it always re-occurs. Had a mechanic look at it and he analyzed engine with computer and found no error code(s). He replaced an air sensor but it had no effect.I have told that maybe its due to the ecu (but they are not certain) and the ecu costs over than € 1000. Any ideas please help
Thanks in advance