I also use a drill and paint stirrer. Always had brown gunk at the bottom. Asked megacrop about it and they told me it was "organics". Which is fine but still gunked stuff up. Somehow everybody here but you and me can get it to fully dissolve
I tried everything from grinding it finer to using hot water and a blender. Only ever tried the 1 part. Haven't used it in years though so don't know about the newer versions
Pulling the pump off the floor or adding an inline filter after the pump should help