When the coolant gets hot it expands. If you have the rad filled to the top the extra volume has to go somewhere, that would be out the overflow tube. Once the coolant cools the fluid level will be somewhat below the top of the filler neck.
Maybe your rad cap is no longer holding pressure and more coolant is escaping than normal.
I installed an overflow bottle on mine to catch and return the overflow back to the rad when it cools, that keeps air out of the rad. If you put on an overflow tank make sure you change the cap to one that will allow the fluid back in as pressure drops in the rad.
Just a thought, I'm not an expert.