Series 3 gearbox problem.

I have a 1973 2.25petrol station wagon.About 6 months ago the clutch started making a whirring noise when depressed.I've been told this is the thrust bearing.Then over the last few days i've had problems selecting gears.Landrover jumped forward when I started the engine in gear yesterday with clutch in.Took it out of gear & started it,then it wouldn't go into any gear.Eventually it just slipped in as normal.Never had any clutch slipping & never had any gears crunching or grinding.Today wouldn't change from 4th to 3rd so had to chug home in 4th.Then completely normal again.Very strange.My mate says it couldn't be the clutch but probably the gear selectors?Has anyone got any ideas or help?Any advice would be greatly appreciated.
Cheers,
Gunwennap.

There may be two problems:

1) Clutch release bearing is worn and needs replacing.
2) 3rd to 4th synchro unit has a broken detent spring giving rise to inttermittent problem?

Clutch plate breaking up maybe?