I had a similar experience to Scott Harvey on a beetle. It was caused by a 
spacer between 3 and 4 gear clusters unwrapping and jamming in the 
selectors. Don't know if later gearboxes suffer this problem but it was 
considered de rigour by the hot rodders back then to weld this spacer to 
prevent it unwinding (It was formed from a flat strip wound into a sleeve).

Trevor Easton 1974 Camper DeLuxe, 1962 Land Rover Series II, 1974 Land Rover 
SIII, pair of Golfs, 1964 Vespa 90SS, 1970 Vespa 200 (now 290) and a ford. 
(Family vehicles)