Elastics are pretty dirty. This is a sign that the whole thing needs cleaning. Assuming it was well-cleaned before it was put out on the floor, that suggests there are dirty spots that are hard to reach, like the below-playfield mechanisms. Give the whole thing a good cleaning and see if you can find the mystery sources of dirt. It wouldn’t hurt to wipe down the easy-to-reach surfaces in below the playfield and in the cabinet, as some of them are filthy.