When my Aqua Pure cell went bad last year I would get crazy readings and false alarms like yours. Most SWCG's only last 3-5 years. That is why if you read the posts on this site you will find that a lot of pool owners with SWCG 's use liquid chlorine bleach. Using the bleach will cut down on how much chlorine your cell needs to provide. I replaced my cell with a Pentair IC40. It is for pools up to 40k gallons. I found it on Amazon for a little over $600.

Any pool store can give you your salt levels. While your at it get a complete water test done and post them. With your SWCG going bad you might find other issues you didn't know about.

Good luck.