I would suspect a dirty throttle plate. They should be cleaned
annually. I wipe mine out with a clean rag and light solvent.

This site has instruction to clean the flame trap. Follow them to
the point where you see the throttle plate:

http://volvospeed.com/Repair/ftrap.htm

Then manually flip the throttle up (or pull on the throttle
cable) and clean out all that black sticky gunk (it can hold
the throttle slightly open).

When you re-assemble double-check the throttle cable is correctly
routed across the platic wheel (you'll know what I mean when
you see it).