slipping clutch or not

hi all
This one will get you thinking !!! i have a 1992 GQ patrol diesel had a new heavy duty clutch kit fitted 18 months a go cost $1500 dollars car has done less than 25000k's since fitting no towing or heavy work i got 250,000k's out of last clutch so don't think driver technique.
Problem driving to the Argadells in the southern Flinders ranges last week, when i tried to over take in 4th gear revs went to 3500 vehicle did not go any where i thought one stuffed clutch. We where close to camp site so kept going ,travelling with club so had plenty of back up.Next morning the mechanic in the club took it for a drive clutch worked fine he said nothing wrong so for three days vehecle went up and down some fairly steep hills not a problem. On the way home started slipping again a very slow trip home.Took it back where i had clutch done answer sorry mate out of warranty not impressed but what can you expect these days ?
Took it to a fifth mechanic today told him that i have had 4 mechanics look at it two said clutch is stuffed and two said it was ok so he was the deciding factor his answer he would not commit either way what am i suppose to do . I am quite good when it comes mechanics on a vehicle but this one has me whacked. I all ways thought if a clutch slips it is stuffed but this one seems to slip then not. All mechanics did the same tests that you would expect
but no joy. Any ideas ideas short of burning it would be welcomed.
thanks in advanced
Al