Had exactly same problem on mine...
...Hope your sitting down. Turns out it was rear main crank seal. When I was using car alot the oil that leaked (very slightly) from knackered seal was enough to make clutch slip but after a couple of hill starts, slipped clutch take off problem when away.... After a few of them slipping went away.
...That is until I stopped using it for a week or 2 then it was back, with a vengence. I tried everything props g'box but endned up been rear main seal. Unfortunatly only real way to check is split gbox and engine and see it there is STICKY oil ming pasted on sides of bell housing (asuming you've not been fording it with plug out...)
Anyway, change clutch while you are there, as its probably comtaminated.
Not a difficult job to do, just a MFer with cold hands (getting spring on seal to go togeather)
PS I left mine for ages and killed the diff... as banging REALLY stressed transmission.
Also if you do it, be careful of slave cylinder bolts, my mamory tells me they are different - UNC and I stripped the casing trying to put a UNF in there (as all other effing bolts on this job are unF!!!!!!!!!!) Might be talking crap about that but there was something funky about them!!!!!!
Max
