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I recently helped a friend refurbish his 81 that had been sitting for a number of years.  Got it cleaned up, rewired, and running.  Ran fine for about a month then stopped cooling.  He found the line entering the evaporator frozen up and the cabinet warm.  Let it sit for about a week and turned it back on to see if it would do it again so he could take a picture.  When he did this, it came on, cooled, cycled, and appeared to work normally.  Any explanation?  I am guessing possible sccale inside the lines that may have created a blockage in the lines, but that is only a guess.  Any input as to the possible cause and more importantly, a possible way to prevent this from killing the compressor?

No expert here, but I know they will chime in.  Seems like there is a small hole somewhere in the line that lets moisture in.  Over time that freezes and blocks the line and it stops cooling.  Unplug it and let it sit and that area thaws back out and it starts to cool again, but again at some time it will freeze up again.   Not a technical answer, but my thoughts as I have had this happen to me.  I took mine to a member on this forum and they found a leak, sealed it, recharged it and I was good to go. 

That actually sounds like a very logical answer. I look forward to some additional thoughts on if this is the most likely cause, and if so, is it something that can be found fairly easily by someone with the right knowledge or will this be a difficult fix

      If the system had even the smallest hole, the pressure inside would deplete all your freon and there would not be any cooling period. But Nick is right,
I'm not sure how, but moisture sometimes accumulates inside these closed systems and when subject to freezing temperatures will turn to little drops of ice.
Take the unit out and have a service technician remove the freon and either strain it or replace it.