Staffs jumping out of gear on braking

I am having a recurring gearbox problem that is not fixed by putting in new dogs. My car jumps out of the usual 4th gear (3rd for me the way Stohr did their shifter) under braking. If I hold it in, it stays, but it is very tricky to do on entry to turn 1 at Road America. I lost a lot of time because of the problem at my races there last year. Does anyone have any ideas what might be causing the problem and how to fix it? Thanks.