2004 Panda 1.2 70Kmiles
The car is in very good condition and has not been abused or thrashed, however there is a problem with second gear. There are no funny noises and the gears all engage perfectly but occasionally it drops out of second gear completely. When this happens the lever moves slightly forward but to get it back into gear you have to move it to neutral and then back to second. This only happens in second gear and has only started over the last couple of days. Looking under the bonnet we can see that the gaiter on one of the the selector cables has split and the shiny steel rod is visible. We've been told that the best way out is probably a new (second hand) gear box but I'm wondering if it could be a gear cable problem. What would be the effect of a stretched cable? Also, if we do need to go down the route of another gearbox, does anyone have any experience of "The Punto Shop" as they are offering recon gearboxes for £275 which seems quite reasonable if they have been properly reconditioned. Any advice much appreciated.

This might help
http://www.wcengineering.com/articles/popout.html

But basically, a gearbox/engine rocking on it's mounts can pop the gear as it pulls against the linkage as it moves.
Or
Worn bearings causing the shafts to move apart inside the box

I managed to sort a Rav 4 popping out of 5th, but the gear and bearing that were fecked were just inside a plate accessed through the wheel arch.

You'd be wise to get it checked by a gearbox specialist.
The only two decent ones I know are in South London, if that's any help.