s2's arent the best for keeping cool, but It could be a problem with your water pump or thermostat housing. Check to make sure both are working correctly. It could possibly be the airtec cooler. I personally ran a pace double pass when I had my s2. Is much more compact and still alows some decent power. Never had overheating problems either.

You also have to remember where the turbo/exhaust manifold is in relation to the rad. That doesnt help the cooling either.