Why is my MJ1200 with SureFlow mod doing this?

[Myka]

Sometimes it fixes itself, and sometimes I have to unplug it. The propeller is sliding up the shaft, and the petals of the propeller are whacking into a tab on the inside of the basket end. I'm thinking I could nip the tab off, but i'm not sure what it's purpose is. Anyone know why this is happening?

I just bought the pump used, soaked it in vinegar, and put it in my tank. It started doing this right away.