chalk up another vote for impeller blockage. do you have a dog swimming in your pool by any chance? my golden retriever's hair - at least some of it - gets past the pump basket and gums up the impeller, which causes a very noticeable drop in pressure.

if you or someone in your house has very long fingers, there's a chance you can reach into the channel from between the area where the basket sits and the impeller. i can on my pump, and almost every time the pressure seems unusually low, i'll find some stuff gumming up the impeller.

if you can't reach in there, you may have to take the back of the pump off in order to get at the impeller.