Samsung refrigerator is fine but freezer warms up after few weeks

Hello,
Was hoping if someone can point me to the right part. This is a bottom freezer fridge. The refrigerator works find but the freezer compartment first fails to make ice and as the days go by the freezer eventually goes warm but the temprature on the front panel says it is constantly at -2.
When opening up the freezer compartment I saw the evaporator coil completely frozen over so it looks like the cool air gets blocked by the frost buildup. After defrosting the freezer and ice maker will work fine for a couple of weeks but the same problem will reappear. Any suggestions on which part to try first would be appreciated.

Thanks,
Ed