I agree with the cell being bad or possiblity low salt levels. I have the same cell/controller with an 18,000 gallon pool and run it at 55%. These are my numbers:

FC 4
CYA 60 (down from 100+)
Ph 7.4
Alk 100
CA 600 (New pool/plaster.. Have to drain/refill to lower)
Temp 92 Full sun all day.

I run it at 5-10% during the winter to keep 1-2 Chlorine level.