Holden Captiva 2009: Towing capacity

When we purchased our new Holden Captiva in 2009 we discussed the towing ability with the sales staff and were assured that buying the diesel was the best option and that it would handle our towing requirements with ease. But the first time we towed with it the temperature gauge went straight to the top and we lost 50 per cent of our power. Holden has since told us that the car was mechanically designed to do this and that if it gets too hot we should just pull over and wait for it to cool down. This is of course not practical or safe. How can we overcome this problem?