thanks for the comments so far. I don't think I need a new clutch because it had a new one about 20,000 miles ago, and I don't have a heavy foot. There is no problem with shifting gears.
The problem lies with the dual range transmission and the transfercase slipping out of High/Low. Every five minutes or so, the lever just sits up a fraction of an inch which is enough to pull it out of gear. I say pull it out of gear, but clearly the problem is something 'pushing' the gear out of gear, not the lever pulling it out.
I don't use the Hi/lo very much, just a couple of times in the snow recently, and have never had any trouble with it before.
The only other thing I can add is that when I push the lever down, it seems to be barely engaged. I'm sure there used to be more resistance when it was pushed home before?
Forester 2000 GLS non turbo