Not enough info, my friend.
Does it slide out on power, off power, when neutral (coasting), what?
Your rear end is obviously losing grip but you need a clairvoyant to guess why (see above) if you don't let us know. The solution is going to be different.
At a guess I would look at roll centres first (admitting you have the spring ratios right, the right tyres, and the rear gear diff and so on, which is all as far as we know a mystery) because of the low grip surface, which makes me think the car needs to roll a bit to put some weight on the outer wheel to plant it down.