Do you have alkaline water or do you add some salt to your pond?

Just a WAG. I had both & would get some rust along the impeller or its housing occasionally. I'd buff it off with fine steel wool then coat both surfaces with that silicone lube stuff.

Are you sure the shaft is seated securely? Or maybe the little rubber buttons at the ends have corroded & are letting the shaft wobble a bit.

Another possibility might be that the plastic housing that holds the impeller in (I've forgotten the technical name of it) can have a hairline crack. I took my pump apart a number of times before I discovered the crack. I bought a new housing and impeller (old impeller was still working but, as you said, not smooth any more) and it works like a charm now.

Another possibility is that some plants have gotten in there. However, if you have a pre-filter hooked to the pump that won't happen. Been there, done that too when the pre-filter came off and sucked in a plant.

Good luck.