Don't see where to post this on the board so I put it here. My rearview mirror is orig. It will not stay in place. it keeps flopping DOWN. It has the orig, I assume, tightening screw that seats down in a hole. I can tighten it like heck and it will hold for a few bumps in the road then it flops back down. The treads on the screw are not stripped, cross threaded etc. IS this common for these at this age? What is the fix? Can't put a lock washer on it since the screw sits down in a cup.