Everything is going wrong on my corsa could it be an earthing problem? ?

I have a 53 plate corsa active 1l. After haing it stood for 6months. it wouldn't start up anymore, we changed the battery and it began starting every couple of times. we have had 5 different mechanics on it now and they have.... changed the ECU, changed the Throttle body, reset immobaliser and have said it isn't the relay. It now starts everytime but only runs for about 20 seconds. the mechanics we hired have more than 50 years exp and have always been very reliable but this time they say they just can't find the problem. we have also had a vauxhall diagnoistic on it and no fault came up could it be an eathing problem or does anyone else have any ideas??