I'm stunned that a 325# sand filter is too small for a 1hp pump and 18k pool!

I suspect you are going at it all wrong because that filter should EASILY be more than adequate, even oversized. I run a 1hp (full-rated) pump with a 200# sand filter and 19,000-20,000 gallons with no problems at all. I have a two speed and full speed isn't a problem.

So...it could be a damaged or defective filter, multivalve, or pump. Or the whole thing isn't plumbed correctly. But there's no way a 325# filter should be too small for any 1hp pump. So I'm thinking you are looking in the wrong place for your solution.