Definitely sounds like the sand is doing its job and catching something that's plugging it. The drop in pressure in recirculate mode is normal...no resistance from the sand media. The 25 psi is probably almost the blocked pressure number. Seems like plugged fiilters are 'in' this season as there are quite a few threads about the problems. One suggestion is to use skimmer socks to trap crud before it hits the filter. A pain changing and washing them so often but beats using precious water to backwash. Give them a try until the crud is cleaned up.
Al