Gearbox problems

[PaceCars@aol.com]

Having a problem with my Ford gearbox. It is a Ford 2000E (1600 three-rail)
in a Mallock U2 with 1600 crossflow. When shifting from fourth to third, most
of the time it balks and won't go in. No grinding or popping, just no
movement past neutral. Sometimes double-clutching helps, but the tiny footbox
makes that difficult. It goes in just fine with the car at rest, so I don't
think it is a linkage problem (?). Before I pull it out, does anyone have any
ideas or experience with this?
Harold Pace