The pitot tube is held onto the wing with four machine screws that protrude through the wing into a wood reinforcement block.  The nuts behind the reinforcement block ride on the wood and there was insufficient bearing surface to avoid crushing the wood.  Thus the tube was always working itself loose.

To fix this problem either use AN970 large washers to back up the screws, although the washers will probably have to be cut down in order to clear the pitot and static tubes themselves, or else build a large backing plate that covers the entire wood reinforcement block.  I did the later using a piece of .040 aluminum, cut to the size of the wood reinforcement block with a large hole drilled in the center for the pitot and static lines.  To make installation easier I installed nutplates onto the backing plate so that I wouldn't have to fiddle with nuts in an invisible and difficult to reach location.

Since installation of the backing plate, the pitot tube has not worked loose in over 100 hours of flying.