Need help - Doughboy Silicia II + Haywood pump - Very low suction (see details)

Here's the details:

12,500 gallon above ground pool in Michigan
Haywood 1 hp pump (Super Flo, I think?) Pump is 3 or 4 years old
Dougboy Silicia II sand filer - Mid 80's - We JUST replaced the sand

BACKGROUND: We've been having problems all season and as soon as one is fixed, another one pops up.

I installed a new "strainer basket" this year. The old basket cover would not seal properly.
After replacing the strainer basket I was able to get good pressure on the pump/filter system (for the 1st time all year)

We've had a "green pool" problem all this year, partly due to problems closing the pool last year and partially due to the pump/filter not working correctly this year due to strainer basket cover not fitting correctly.

We finally got the pool blue again, but the sand filter was so contaminated it was putting algea back in the pool.

Yesterday we took apart the Doughboy filter, rinsed it out, and completely replaced the sand with 125 - 130 pounds of new sand. This is the recommended amount.

PROBLEM: Now the pressure is VERY low when running the filter. 5 PSI or less. This is exactly the same pressure we had before I replaced the strainer basket to fix the bad strainer basket cover seal problem.

I'll prime the pump by removing the strainer basket cover so the basket fills with water. I'll replace the cover leaving just a small air bubble at the top.

When I start the pump the pressure will be high for a few seconds, but then all the water in the basket will be sucked in, and the pressure drops suddenly to 5 PSI or less. The basket will no longer be full of water (looking in through the clear cover).

Suggestions?