I got my Standard off Ebay used a couple years ago. I love mine and don't plan on ever changing to a different vise.
If your head is rocking when you push down on the cam lever perhaps you're trying to over-lock the hook into the vise. One thing I notice is if you don't put the hook into the jaws far enough or "deep" enough you might think you have to really crank down on the cam to lock the hook. You don't - you just have to get a little more of the jaw surface on the hook.
Also make sure you're using the right jaw for the right size hook.