Bottom freezer door dripping water on kitchen...

Question

I have a Sub Zero Model 550, refrigeration on top, freezer door on bottom. Unit is 13 years old. Recently found water dripping on kitchen floor from freezer door. Freezer door gasket was replaced. Still having the same problem. Water is puddling on bottom of freezer compartment. It is freezing then thawing and dripping from bottom of door onto floor. What could be causing this? Any help would be appreciated.