This is a well known problem.  Routine dealer maintenance should prevent
the problem.  The solution is to clean the throttle body.  A buildup of
deposits causes excessive friction with the throttle lever.  Our dealer
has further found that adjusting the throttle so that it doesn't quite
go completely off will help prevent the problem.

It is a very dangerous problem if one uses cruise-control.  What happens
is that when you go down a hill with cruise-control on, the throttle
will go to the off position. Then at the bottom of the hill the
cruise-control will not be able to resume control until the car has
slowed down enough to cause a large force on the throttle which causes
very high unexpected acceleration back up to the set-point when the
force overcomes the sticking. It is scary.

I have a posting on this on my Volvo website.