I had a similar problem with my sporty - a mate advised a good service, making sure you change the air and fuel filters. If it's hunting, it's either having trouble breathing, or doesn't like the fuel. Try one of the fuel additives you can get from Halfords, and maybe treat it to a tank of decent petrol - clean system 3 or somesuch - the reason the supermarket petrol is cheaper is that sometimes it just isn't as good as the others.
Good luck!