It sounds like the same problem I have with my '84. I've gathered from the
list that it likely is the flywheel that is warped and needs to be cut. It
happens from riding the clutch too much or a broken seal leaking fluid
onto the plate and making it slip.. I am letting my go until I need
to replace the clutch and then replace seals,plates,cut the flywheel etc.
I also have no trouble driving or shifting, but this only happens in first
and reverse.

My buck $2.80's worth
Steve
'84 westy
'95 jetta