I had the same problem and it was the rubber gaiter on the stick
shift that was getting trapped between the edge of the hole in the tranny
tunnel the shift lever, when slowing down using the gears the lever would
throw out, not every time but often enough! It`s worth a look there.
>>>>>>>>original message follows<<<<<<<<
>I have a minor problem with the gearbox of my '69 B: if I want to slow the car
>down by releasing the accelerator while engaged in third gear, it wants to
>throw itself out of gear into neutral. Otherwise the gearbox is smooth. Any
>ideas for possible causes and what it would take to fix it?
>Frank van Dalen