Could you tell if the engine turned over a little?  Some times the vehicle will
move a little and then stop when compression builds in the cylinder.
But it should hold on a slight incline. 

How old is your clutch?  Check for any fluids at the bottom of the bell housing.
You could have worn seals and some fluids leaking onto the pressure plate and
clutch. 

With the clutch depressed you shouldn't have moved.  Maybe your 
slave cylinder is starting to fail.  This would release the clutch and it might grab.
That doesn't explain why it would stop grabbing though.  Changing 
the clutch isn't that bad of a deal.

>>> John Schaper <JSchaper@SCHSA.ORG> 02/20 9:31 AM >>>
1985 1.9 Vanagon.  Been running great, but noticed something over the
weekend.  First, when parked on a slight incline in first gear (manual)
without parking brake, the van tended to slip backwards.  Second, when
stopped at a light, clutch fully depressed, I noticed an occassion
"surging"--kind of felt like the van moved an inch or so forward.  I don't
think I'm making things up, but am concerned that this may be a prelude to a
problem.  Should I be concerned or not?