Viking Imperial Fridge... Purchased in early 90's

Question

This fridge is like new. Except........after 6 to 8 weeks, the fan in the freezer begins to roar. Like the fan is catching on some thing. (ice I believe) The back of the freezer can freeze up to over an inch.

The freezer stays cold but once the fan begins making the noise, the fridge slows and then stops cooling.

We remove all contents and let the freezer thaw out for at least 8 hours.. plug back in and it works like new again. Any thoughts on what is causing the problem? Thank you