I have been cutting out some puzzles recently and noticed right off that my table was a bit out of square.  I got my model builders square and trued it up, but it wont stay vertical.  I have strong fingers, in my younger years my nickname was, "Vice Grip Fingers", but no matter how much pressure I put on it it still moves enough to make the puzzles hard to put together.  Short of getting out a pipe wrench, I don't know what to do.  I cleaned out the sliding area, and tightened the bolt in the back, but it still wanders out of square.  Anyone else have this problem, and what can I do?

Randy