High flow, then low flow with DE Filter

I have an old pool in a house I moved into 3 years ago. I have done most of my own maintenance and usually can get my pump and filter up and running without any issues. At the end of last season, after disassembling and cleaning my filter, I noticed that when I started everything back up, the flow was nice and strong for a few minutes, but then would drop off significantly. I just let it go and over the winter, the pump has been running for a few hours per day, but I haven't been doing much else. I figured my filter would be filthy by now. I went out to try to get things back up and running and when I backwashed, it was not significantly dirty. When I got things running again, it did the same thing. High flow and normal pressure for a few minutes, but then the pressure increased and the flow went down. I figure it's time to open it up and clean it out again, but is there anything that is going on here that sticks out as a possible solution?

That's a pattern that we've seen before, but not with any single obvious cause that I can remember . . . except algae.

Live algae plugs DE filters almost instantly -- if you have algae present, kill it first.