What's wrong with my refrigerator

Hi…New to the forum & hoping someone can help.

The Freezer gets close to freezing, but not quite, & the upper right of the cooling coils ices up. It does go through a defrost cycle, & the defrost Heater does come on/cycle normally, and also when I advance the defrost timer by hand. The defrost timer does make a constant "quiet" clicking sound as it runs.

Been Checking it for several days, & it almost makes ice (top layer of water in the ice tray freezes, but below the top layer there is water)

See the attached photo. A larger photo can be seen here:
_MG_5974Large.jpg photo - nidoba photos at pbase.com

I do have a new Defrost timer, but haven’t put it in. My suspicion ('cause the current one seems to work) is that is not the problem. Looks like a thermostat of some sort gets caked with ice in the iced up area and may not be sending an accurate reading to at some point to control somewhere.

Any help would be appreciated.

Thanks, Mike