You probably have the same problem that I had last month when I hit the Arizona state line. There is a small hose under the intake manifold. It is like a small elbow. There was a split on the inside of the hose. This would only leak when the cooling system was under pressure or in my case, overheated. You have to remove the starter and some other patrs to get at this $5.00 part. It takes about 4 hrs of labor. The best way to test for this is to pressurize the system for a few hours to find the leak.
Hope this helps,