Sorry to say, but this is most likely a badly work synchronizer or bearing. Either requires trans removal and disassembly.
One other possible, but slim chance, is the clutch adjustment not fully releasing and placing a load on the gears when stopped.
This trans was used from 49 through the mid - late 60's (with a change to open driveline in 55)when it was replaced by a new Saginaw with all 3 speeds synchronized, so parts or a used replacement should be easy to find.
