My refiridgerator has been freezing up, Reading up told me the defrost timer may be bad.

First I removed the two screws that hold the timer cover in blace, then I removed the the two screws that hold the timer onto the cover. I was then able to unplug the timer, and reverced the process to re-assemble the timer. Unfortunately.....replacing the timer, did not solve the problem. My refridgerator still freezes up every other day or so. It will eventually thaw, and work for a few days, but then freezes up again.