I have the same issue, except my instant salt readings alternate between -2000 and -100, even though the salt concentration is 3400, as measured independently by my pool technician. Since the cell is 10 years old (only runs 4 months per year/12 hours per day at 30%), I am assuming it needs replacing and I have a new one on order.
My question is this - can I still run my SWG until the new cell arrives without any damage to the controller? I have been able to trick it into working every day by recalibrating the average salt reading to an acceptable reading at start up. I then run it at 100% until the average reading falls to a low level and chlorine generating stops - this usually takes about 4 hours, which is sufficient time to sanitize my pool. I dont want to do anything to damage the rest of my system while waiting for my new cell to arrive.