I have to change my front pads like twice a year because of vibration and shuttering even though the pads are not badly worn. I have turned or replaced the rotors several times over the years and am aware of the importance of properly tightening the lugs but nothing seems to help. I've also replaced the calipers in the past. Fortunately I have a lifetime warranty on the pads so the cost isn't painful but I'm sick of changing them. What am I missing?