Gearbox pops out of third gear on track

[Joepster]

On track, under heavy braking, my 172-1 pops out of third gear.
The gearbox itself is revised, i have all mounts upgraded, tried a heavy gear knob ... ... really tried everything but cannot solve the problem.
Anyone any idea?

 

[brisbane956]

Did you ever get to the bottom of this, I’m having the same issue! Mega annoying!

 

[Dawnspinner]

Had the same problem on our phase 1 track car, but it was 5th gear jumping out. Asked around and was told it's the synchromesh just held it in when needed . Anyway last outing to Cadwell on the bottom straight co owner puts into 5th bang!!!!!!""***~~. Towed back , no drive horrid noises from gearbox. Got the gearbox out end cover off ooooh! No teeth on 5th, synchromesh gone and loads of bits. Hope you've had a look be warned.