http://www.youtube.com/watch?v=5Emp5vLwDmg

As the video shows, my car won't stay in 5th gear. after googling, I've returned quite a few results on why this would happen, however, I don't know if any apply to the applause.
(As all the other info I found came from Subaru and Suzuki sites)

a LOT of people were saying Synchro's
(Then one bright spark said, its not the synchro, as that is only used for going in to gear, not STAYING in gear)
Someone said a nut on the gear had come loose. (Subaru Impreza though)
and someone else said there's a spring that holds the gearbox in gear, which could have snapped (Suzuki)

The problem is, all these results were for other types of car, as my googling was unable to uncover any Daihatsu related gearbox not staying in gear issues.

Any ideas on what would cause this and how to fix it?