Nifty video, it does a great job of showing what you're talking about AND entertaining me, cool. The most likely (100/1 odds) cause of this is simply that your board is too soft, if not overall than definitely between the feet. If you are sentimental and just love the board than I would recommend getting a hold of a VIST plate form Allboard sports in Boulder Colorado or from Hardbooter.com, they both have them and charge about the same. If the $550 or whatever it is (remember the VIST will far outlast your board and is easily moved to a new one) does not sit well than it is time to check out a new deck. I recommend checking out anything you can get you hands on that made with a PROGRESSIVE flex not radial, that is stiffer between the feet getting softer towards tip and tail. Any of the semi-custom builders can do this and if they have any sense at all they will be happy to do it for you. I hope this helps.
Adam Casanova