DE blowing back into pool

Good Morning.
As this is my first post to this forum I will start with the most troublesome problem.
We have a Hayward DE4820 filter, with a multi-port valve, Jacuzzi pump (don't recall new motor at the moment), Hayward inline chlorinator on 20x 40 inground pool.
We have DE blowing back into the pool. We have replaced all the grids with Unicel grids, replaced all O-rings on everything and a new multi-port valve. We are still getting earth when the filter is turned off and back on.
My husband wants to buy a new filter, but before we potentially waste money I was thinking we should try to replace the manifold and the outlet elbow and inlet diffuser. We see no visible signs of cracks. I have read a lot of posts and advise but have had no luck solving this problem. Thank you for any help you can give me.