Vauxhall Vectra no response to throttle pedal

My 2001 2.2 Vectra (petrol) has done 54K miles. Recently on starting the engine ran but did not respond to throttle, cutting off after 20 seconds. This happened several times in the same way.
The computer diagnostic from the guy that maintains the car indicated a fault with the throttle body. When this was replaced with a new part the car ran perfectly but only for 4 days when the original fault reappeared.
Further diagnostics show normal voltage levels at both the throttle pedal and the new throttle body when the ignition is on and do not register a fault code. As soon as the engine is started the same fault code 5114 shows and the engine shuts down after 20 seconds. Whist running it does not respond to the pedal.
Follwing this the ECM was changed, all was recoded and the car ran perfectly but only for 36 hours when the original fault returned again.
Any ideas - apart from scrapping it - would be welcome