OK, now we see that my alk and CYA are a little low, but not super low (except CYA). I will work on those two tomorrow. I don't think I have lost my sand, but I have plans to take the sand filter apart tomorrow. The dP on the sand filter rises generally over time, as you would expect. I backwash normally ever 1-2 weeks, and you can tell if the filter is getting plugged because not enough water is coming through the pool return jets.

Thanks for your advice and input.