I would try another lab, and talk to the first.
I have a friend who runs a mini lab at Campus Camera, and they clean the rollers daily or anytime they see a scratch.
Some mini labs have rollers that get swapped out on a regular basis, and then out there, there are a lot of variables in other labs.
Scratches on the base, sounds like pressure plate, I would put a loupe on it to begin with. If you can overlap your negatives from different rolls and they match up, it should give you an idea where to look, obviously don't forget the image is "upside" down as are all real images.
Regards, John