My 10 year old Sub-Zero refrigerator works very well except for two issues: 1. I continuously have from 1/2 to 1 inch of frost build up in the center of the back wall. I have cleaned the refrigerator including the drain tubing in the lower area. I have also had the gasket changed by an appliance repair technician. 2. It may be a result of the frost issue, but I now have rust developing on the lower front lip area where the door closes. I have cleaned this area several times and dried it after each cleaning. Additionally, when the refrigerator was first installed the door was considerably harder to open than it is now. I thought this may be because of the seal/gasket??? I hope you will guide me to the solution. thank you in advance. Teresa