Classic/Antique Car Repair: 1950 transmission problem, transmission repair shop, highway speeds

Question
any idea why the tranny in my 1950 chev tries and kick out of third gear at highway speeds..? I can hold the shift lever and it will stay in gear, but still tries to kick out. Linkage adjust, or something much worse..?

Answer
This is what happens when the mainshaft or the bearing for it is worn out.  There is too much play in the mainshaft, and the gear is not running exactly perpendicular to the mainshaft.  There is no adjustment for this, the transmission has to be repaired.  You can remove it yourself and bring it to a standard transmission repair shop, or if you have some experience and the special tools required, you can do the repair yourself, but it must come out of the car to do this.

Dick