jumping out of 4th gear on hard deceleration

[65Austin]

1965 998 cooper with remote shift (original setup, 3 sync)
is there anything else that "locks" 4th gear on other than the 3rd/4th shift rod plunger and spring?
i have replaced the nylon bush and coller on the shift lever (lots of grease doesn't rattle) cleaned plunger and spring on the remote end, replaced plunger and springs on the box, checked that the plunger goes "in" when 4th is selected, had the knuckles off checked spring/cup and grease on those, reassemble and checked the alignment, it shifts great on all the gears including 4th (shifter is not sloppy) and can hard decel on 1,2,3 no problem, but 4th pops out on hard decel, seems ok on lower revs in city driving, it is possable in city driving that some engagement points on 4th are better and seem to hold the gear better but i can't be sure, might be my imagination, motor mounts are good, new rubber mount on the remote, had tried moving the knuckle joint by a spline or two either direction but it doesn't make a difference other than change the location of the shifter in the car more forward or back. trying to elimanate everything on the outside, running out of ideas...

thanks

[2ramp3]

On acceleration the torque produced in the mainshaft causes the gear and syncro hubs teeth to pull towards each other on deceleration that force is slightly less so more emphesis is put on the gear locking mechanism ie the gears dog teeth and syncro hub the first sign of wear on gearbox gears is normally jumping out of gear under deceleration , the ball and springs you mentioned are for locating the gear change not locking the gears . i think you need to check in the box itself for cause if you have ironed out any external reason for gear lever movement .ps the engine and box would need to be litery falling out of the frame to affect gear selection and driveability despite what people may have thought of bmc and it products the A series engine and box were bullet proof in any vehicle but not immune to wear and tear.