If the gear shift retainer becomes loose on the pins, it will allow the lever to pop out of the slot. If you can easily depress the retainer and turn it, it is too loose. This is a very common problem with the T-176/7.
There is also an e-clip located externally on the shift tower that secures the linkage section that is depressed when you shift into reverse. If the clip is missing, the linkage bumps the shifter whenever you go for 3rd and 4th. This normally does not push the lever enough to dislodge it from the groove, but it can assist in wearing the groove.