I'm not familiar with the Getrag, but on general knowledge . . .
Often there are two similar mechanism: detent and interlock.
Detent holds the shift mechanism engaged or disengaged -- in gear or neutral. Typically, detent comprises a spring-loaded ball that registers into a notch in the shift rail. Detent can be overriden by pressure on the shift lever, or by unusual axial forces on the shifted gear hub.
Interlock prevents engaging two gears at once. Interlock may take many forms, but often comprises a round-ended plunger between notches in two shift rails: when one rail moves into gear, it forces the plunger to engage a notch in another rail, so the second rail cannot move until the first rail returns to neutral. Because interlock is solid -- not spring-loaded -- it cannot be overriden without serious damage to the transmission.
Spontaneous disengagement may be caused by:
> Detent ball not moving in its bore (This was the case on my Isuzu.) Distorted detent balls could cause this problem.
> Detent spring weak or broken
> Worn detent notch in the shifter rail
> Worn shift fork (a problem in my old Opel)
> Worn synchronizer/hub, causing excessive thrust against the detent
> Maladjusted shifter
> . . . and maybe other things that I haven't thought of
Clutch pedal or hydraulics should not cause the transmission to jump out of gear.
My custom flywheel was cut down too far and now can't be used in my 3.8 conversion.
