Clutch slipping on Defender 90

Hi,
Has anyone had the same problem with their clutch on Defender 90, 1985 mine was put in November 06 and has been back in the mechanics twice with the same problem, which is:
When the temperature gauge reaches half way/stuck in slow traffic jams or just run a long time the clutch starts slipping.
I just need some idea of why? so i can confront the mechanic and know what to ask him to do to fix the problem.
Any help appreciated.

Regards Landroverfamily.

Could be all manner of things, but to start with do you excessively "ride" the clutch? if you do then don't, and just for a laugh measure the girth of your left thigh and compare it with your right one , is there an oil leak from the crankshaft oilseal, showing at the drain hole in the bottom of the flywheel housing?