the scenario itself seems just like what some tracks would experience with new (old style) carpet. irp raceway would be that way with every new track change. and i'm talking we wouldn't be able to get a full 5 minutes in on it until the qualifiers had begun.. on the second day of racing. (even after the place had been around for over a year or two). in those cases, the way it was fixed was when all the loose fibers had been moved out of the way (by cars running a decent line).
how long has that layout been down, and how much traffic are you getting in an 8 or 10 hour race day? either, don't vacuum (the groove), or do it as ea suggests. it should get better as traffic starts to clear away the weak fibers from the groove.
as far as setup tricks go, i dont have any. we always waited for the track to 'come in', putting in only 2 or 3 minute runs until it did.