When you pulled apart the valve, the pie-chart gasket should have matched EXACTLY the layout of the ports in the valve. If it didn't then only two things are possible: 1) you installed the gasket wrong.
or
2) You were sold a gasket that was wrong or damaged.

The only other cause I can imagine is a something broken in the 6-way valve.

Another problem you have is your pump is FAR too big for your filter. I have 200# sand filter and I use a 1hp 2 speed pump for a much bigger pool--nearly 20,000 gallons. I believe your 2.5hp pump is exceeding the capacity of the filter making it impossible for your filter to filter properly. Your filter may WELL be damaged by this pump. Cloudy water would be the result. Your best bet is to ONLY run the pump on low speed, never on high speed, even when backwashing, and consider down-sizing it. The savings just on electric should pay for it in a season or two.

Bacquacil: I'm know JUST enough about it to stay away from it. Converting will be a painful process requiring boo-coo gallons of bleach. It may be easier if you first do a 100% water replacement using Poconos's Plastic Sheet method (This prevents your pool from EVER being empty and ruining the liner).

Those are my suggestions.