confusing readings on 4 year old SWCG

Did you ever try acid washing the cell? When you said you cleaned the cell originally I assumed you meant with acid. Acid washing is something that you're going to have to periodically do. There's even instructions in the aqua rite manual on exactly how to do it.

If you didn't try the acid wash, and haven't disposed of the cell then keep it as a backup.

Yes, I did the acid wash but it didn't change anything. I then got the new unit installed and have had no trouble with it since.