Not here to scare you, just additional info, because the internal issue is obviouslu big $.
If the linkage is adjusted properly it should not slip out, this is simple. There is a slight detent, bump ,felt when the selector shaft is in the correct position, one bump for each position, L, H, N etc. Check the position of the selector handle is in the grove which it belongs in. The link rod should line up with the pivot points.
If this is all ok, I'm afraid you may need to look internally.
If there is a rounded dog on one of the gears or a bent shift fork this will allow the gear to slip out of position (or the bent fork never put the gear in the correct position) . If this is the case, when you adjust the linkage it may be ok until the load on the faulty component allows the gear to move out of position.
When this happens it will further damage the internals and likely goon up your adjustment as the linkage was forced from it's desired position under load.