Cutting power from petrol pump (Vauxhall Corsa, 1998, 1.0)?

After numerous amounts of times in the garage my car (Vauxhall Corsa 1998) is still not working. It works for 15-20 minutes then just as quickly it stops working altogether for 15-20 minutes also, this cycle goes on without stopping.
I've tried everything, is there any answer as to why it keeps losing power and dying?