It's important to check the condition of the small teeth that lock the gear to the synchromesh (outer) sleeve and the corresponding teeth inside the synchromesh sleeve. These teeth are what connect the selected gear to the mainshaft and keep it engaged!
You heard me right! The selector fork simply moves the appropriate synchromesh sleeve in or out of engagement with the required gear. The detent springs and balls simply hold the selector rail in the correct position. Neither the fork, springs or balls will hold a gear engaged if the above mentioned teeth are worn. If the profile of these teeth on a gear is worn, the gear will tend to drop out of engagement.
Incidentally, the sound you hear when you crunch the gears when engaging a forward gear is the above mentioned little teeth being bashed against each other (not the actual gear teeth, as many people believe) when the synchromesh unit isn't working correctly or you're being ham-fisted.. If you hear a crunch when engaging reverse gear, you're bashing the actual teeth on the reverse idler pinion against those of the two reverse gears.
Unfortunately, checking/ rectifying the above requires removal and strip-down of the gearbox.
A worn selector fork can cause a gear to jump out of engagement as it's being engaged because the S. sleeve is not being moved far enough to fully engage the gear. Wear in the remote gear linkage or base/mounting of the gear lever on early 4 speed boxes can cause the same problem. Check that the engine and gearbox rear mountings are good. Check that there's nothing preventing full gearlever movement, e.g. displaced rubber boots, soundproofing/insulation etc.
The reverse detent balls and springs give very little trouble. Only problem I can recall was a couple of cases of balls sticking in their guide sleeves due to water ingress/condensation causing a little corrosion (shouldn't happen because there's a gasket under the cover plate!). Or people getting things wrong when assembling the gearbox....
P.S. I rebuilt these gearboxes while employed as a Mechanic at a Fiat Main Dealership, back in the day..... Never had a problem with any I rebuilt...
Al.