This came up a couple of years back. Do a search here. I posted a reply there, detailing how it may just be an evaporator coil in the freezer section clogged with ice, overwhelming the defrost cycle. You'd still get some cooling, but nowhere near enough. The fix would be a manual defrost, perhaps with the help of a hair dryer to speed things up. The cause could be too much heavy stuff loaded against a panel covering the coil, especially if it's an aluminum panel. If I find that thread, I'll post a link here.
Edit: here are two links for you to read:
http://www.winnipesaukee.com/forums/...erator+defrost
http://www.winnipesaukee.com/forums/...erator+defrost