You didn't indicate that it is always happening on the same wheel or not, but if it is, I'd look closely at that hub/wheel/suspension to see if there is something putting extra stress on that particular wheel or maybe even see if there is some maneuver you are performing when driving that may be stressing that particular wheel. If it is happening on both rear wheels, I'd agree that it is probably an overtightening situation. Once you verify the lugs are the proper ones for the wheels, I'd probably back off to 80 or 90 lbs and see if it continues to happen.