I have an older Bogen Manfrotto 3021 tripod, and in many respects it is a great piece of equipment to use. BUT, the cams that lock the legs in extended position have a nasty way of slowly unlocking on their own, allowing the legs to slowly shorten, unevenly, threatening to dump the camera. I've tried cleaning the plastic cams with alcohol and adjusting the tension on them, but nothing so far has resulted in a reliable fix. Does anybody else have experience with this problem? Any suggestions for a fix?