Probably the first thing to do, Peter, is pull it apart and check that the roller tracks on the flat of the focuser tube are free from any built-up dirt or grease. They should be dry and scrupulously (well, that may be going too far
) clean. The roller bearing' outer surfaces themselves need a good clean as well. I find that the rollers can "rise" onto a dirty bump and affect the focus lock right at that point - by moving when you lock.
When you move the focuser along entire path under nice even tension, is it liquid-mooth or a little jittery?