GE side by side refrigerator; frost-free freezer freezes up.

I have a GE frost-free side by side refrigerator with ice maker. It's 7 years old but for the past six months I have been having problems with the freezer building up frost around the inside coils, and then the refrigerator doesn't get real cold.

Three months ago I took the inside panel out, defrosted the coil with a hairdryer and reassembled. I just did the same work again today as the coil had about 1/2" ice build up.

The model number is: GSS20IEMA MW Serial #: FA247490

My questions are easy (the refrigerator is only used to store frozen foods and to hold pop, beer, etc... ), 1) what could be the problem, 2) what would be an estimate to repair the problem and 3) is it worth fixing or should I invest in another new refrigerator?

Cheers,
Chip