Hi,
I'd be looking at operator error before looking at detent spring pressures , If its happening on both bikes and the same jump,
Try these ,
Remove the gearshift lever, after first selecting the gear you think its "falling out " of and try the same jump, yes you will have to push start it ,
Try the same jump on both bikes, wearing your joggers, not wearing your new fangled modern plastic covered boots , you won't be able to feel your way through the gearchange with those on
Try an early Yamaha lever, the after market steel ones ,keep the same length but shorten the part that contacts your boot to 20mm that will mean you have to "feel for it " when changing gear, rather than the gearlever finding your foot/boot by mistake,
My CZ gearshift problems went away when I wore SIDI VMX boots and I used a modified Yamaha gearlever shortened at the bit that catches on your boot
Cheers and
Merry Christmas