oldtelefart

I have a Poolrite Enduro EN850 filter. System is 6 years old, I use dissolved granular chlorine. I backwash and rinse whenever the gauge reaches the red zone. Normally the gauge then drops to the high end of the green zone. The last few days the gauge has suddenly gone to half-way of red zone. A long backwash, till sight glass ran totally clear, made no difference. The needle doesn't drop from the red zone when system is off. Running in recirc, pressure drops into green, no noticeable difference in pressure/flow from eyeballs. Reset to filter, pressure shoots up into red. Any ideas whether I just need a new gauge or do I need to wash or replace the sand?