Sand Filter Issues

[Poconos]

Did a Google on that filter and it looks like you can take the valve apart easily and far enough to check the internals. It looks similar to the Haywards. Put the handle in a middle position, between settings. That relieves pressure on the spider gasket so the halves won't spring apart when you next remove the 6 bolts that hold the top and main body together. The top should lift right off. Shouldn't need to take it apart further to see if the spider gasket or the valve pieces themselves are damaged.
As for the problem of the flow decreasing, I can't explain that yet. The leaking out the waste line is as Carl suggests. If that valve has a recirculate or bypass position I'd try running in that mode to see if the flow drops. This mode bypasses the filter. If the flow symptoms still occur then it's something else in the system causing the problem. Quite possibly the pump. Let us know what you find and we'll take it further.
Al

Edit: Also, what happens to the pressure reading when you switch to high speed and what's the pump rating?

[CarlD]

Al is far more knowledgeable about the multi-port valves and the workings of the pumps than I am so I'll let him take it.

I've only had my MPV apart once and never pulled apart the pump...It ain't broke so I'm not fixing it!.