Dave,
What kind or pressures are you running? The last line in your last post says the pressure starts dropping after a couple days and that would be odd. When the filter starts plugging the pressure rises. Not sure why the filter doesn't seem to be working as well as it originally did because sand doesn't go bad unless it gets gooped up somehow. My sand is starting it's 10th season. The 150# filter for the 1 HP pump is not a good match. Pump too big and/or filter too small and that may be your problem. Trying to ram too much water through the filter. I agree with Watermom about the cartridge filters. This forum is full of complaints about the maintenance and costs. My own opinion is they would be OK in a spa or hot tub where you filter a small volume of water that doesn't get too dirty, but a big pool...no. Watermom is getting away with a 1/2 HP pump for a 24' AG and seems to be working fine. Think she has a 200# sand filter. If you do decide to stay with a sand filter there is a trick of adding a small amout of DE to the thing to get better filtration. The small amount would not cause a backwashing problem as to where to dump it.
Al