I'd start by pulling the intake and doing a real good inspection of coolant sealing surfaces and where the intake gasket meets the heads.
I had a situation on my motor where the heads had been milled so much that the coolant passages didnt match up with the bosses on the intake enough to get a good seal, even after milling the intake!
I ended up having some extra material welded on the botton of the intake to correct the problem.
I'm not saying this is your problem but its worth a look and pulling the intake is a good starting point.
Goodluck, Cp