If its not the lock nut like Bill says, it may be you have a bad motor mount on the same side as the lock nut. I got a 52 here that did that. Every time you pushed the clutch down it moved the motor a little, and took the adjustment out.

Jeff
1947 Chevy 6400 2-ton
In the Gallery
1953 3/4-ton
In the Gallery
It was fun while it lasted