Sounds to me like the pressure plate isn't applying sufficient pressure, but why it would be OK for 60-70 miles is beyond me. The clutch stack height is pretty critical, are you sure you have enough plates etc there.
I had a similar thing that I had left 1 plate out, maybe 2, and was OK for a good few hundred miles, but as soon as the plates started to wear slightly there just wasn't enough depth in the clutch pack to stop spinning under such load, and I ground to a halt I needed to add plates for it to work correctly.
Maybe the pressure plate doesn't fit flush with the plates? maybe the bearing in the pressure plate isn't a flush fit.
I assume you cannot move the clutch hub when there are no plates in there? and that the spring washer etc is nice and tight???
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