How can i trace the ignition fault on my 93 Nova 1.2 spin?

The engine starts straigh away from cold and runs perfectly for about 20 mins. Once up to temperature the orange light comes on on the dash and the engines stops. Also if the engine has been run and then turned off and left for a few mins, it wont start again until it has completely cooled. After tests it is evident that there is no spark present, I have tried different coils, ecu, Distributor etc but am now struggling for idea's.