It's a sand filter, Tagelus, which we changed the sand in about 2 years ago. The normal pressure (in previous years) was consistent around 15, and that's what it's showing now (I just bought a new pressure valve to be sure).

I turned the filter to recirculate and yes indeed it does seems to be moving the water faster and there's greater pressure in the skimmer basket (not a lot, but definitely a discernable difference). One funny thing (maybe) is that on recirculate the pressure reads 12 instead of 15. And another thing: the pressure went up to 25 just a hour ago, after only 2 hours of filtering. I haven't seen pressure that high in several years. I backwashed and it went back to 15.

The pump is a Pac Fab at 1hp.

So, does this sound like a clogged filter? Could it be a leaking spider valve? Or the sight glass maybe leaking? I'm totally befuddled.

Thanks for your help :-)