flow goes down overnight

i have a waterway millenium pump and filter. i replaced the 400 lbs of sand this spring. i also had to replace the "top" of my filter (the handle part). my salt water ate at the valves and it was hissing and making noise and losing flow after 30 minutes. last year, the motor was getting hot because of this issue. my motor has 2 speeds. i used to run on low. this year even after the new sand and pump top, i have to run it on high or my salt generator says low flow. we've been up and running about 3 days and every day i've had to backwash...this isn't normal. all traps and lines are full of water but i'm not sure what is going on. is it my pump?