It's good to grease the spider gasket, but that should not be the problem. Sucking air occurs somwhere before the pump. Once the water leaves the pump it is being pushed under pressure and sprayes through any openings or breaks. Low water or an obstruction in the skimmer could cause the problem. Sometimes the flapper sticks and causes a wirlpool affect. If a cleaner is hooked up a leak in the hose could be the problem. With any of the possible causes you should see air in the hair and lint basket.

When you shut the unit off and let sit a few minutes then restart, does the pressure pick back up?