Pressure just builds......

[waste]

AJ, here might be the problem, Hayward makes some of the best equiptment out there, unfortunately they don't know how to use it. To backwash your unit correctly:
(Obviously, turn off the pump when changing multiport positions)
Put the multiport on backwash, and watch the sightglass for ~clear.
Rinse the filter till it is ~clear.
Repeat until the 'backwash' cycle is ~clear and rinse again. (~3-5 cycles)
Put it on filter for ~ 1 min. and do another backwash/ rinse cycle.
At this point, you've gotten ~80% of the de out, so only add 80% back in, in your case, it's a little under 4#.
Having way too much, or too little, de in the filter can cause the problem you are experiencing and hurt the unit. Try this a couple of times before you replace the grids.
Good luck in your new home - don't forget to spray-paint 'www.poolsolutions.com' on the livingroom wall, so the new owners know how to take care of the pool -Good luck where you are moving! , if it has a pool stay with this site. Again, Good luck!!