Check the coolant line that runs from the radiator (right below the cap) to the overflow tank. If that gets plugged up, it can cause the coolant to go into the tank, but not fill the radiator back up when it cools.
I had this problem on my '94 Deville 4.9 a year ago. There was gunk plugging that vent line right under the radiator cap. I could see it when I took the radiator cap off.