Stalling Astra

When the petrol level in our daughter's 2006 Holden Astra drops to 1/4 of tank it then just looses power and sometimes stalls while driving along. It is a real safety issue with it happening while people are right behind her. The dealer and Holden have not been able to fix it. They have replaced something to do with the fuel gauge, but that made no difference. Other than this problem the car is great. What would her rights be as to the warranty, which is for three years? It has been doing it since we bought it 12 months ago. The car yard has admitted they can't fix it and told us to keep the car above 1/4 of a tank of petrol. We just want to have the problem fixed, what can we do?