Usuallly when a clutch does start to show signs of being unable to cope, then they do only tend to slip under high load - i.e., in higher gears.
Recently I had the same problem after I upped the boost following going BPU.
Up to 1.0 Bar and no problems at all, but whilst accelerating up a hill in a high gear at around 1.3 Bar, the clutch just started to slip, so I eased up until it was sorted. (Driving with it constantly slipping at high rpm will quickly wreck a flywheel - about £400 + VAT)
I jumped in at the deep end and my solution was to replace the complete clutch with an uprated one, the flywheel, spigot bearing and oil seals - not a cheap job.
On reflection, I think I could have possibly got away with just replacing the stock friction plate for £100 or so; that 'may' be a relatively solution for you also...