I dunno if someone have said this before but I've read trhough the posts (fast thought) and couldn't find anything

1 To litle of air in your tires. Probably not since you use a lbs-inflator.
2 Worn out tires check the tires (like judge possener said) to se if your skids has teared down the tire anywhere so there is smal holes for smal rocks to get through the tire.. Dunno how to put this (no native speaker) but i hope You'll understand