The throttle body cuold be dirty causing the throttle to stick
near the closing position. A lot of gunk can build up in there and
they should be cleaned annually. To clean:

You remove the cold air intake hose to the air filter
(it pops out); remove the black cover over the throttle (a torx
screw as I recall); remove the intake hose to the throttle body
by taking off a couple of clamps and then you can see the throttle.
I've cleaned mine out using a rag and rubbing alcohol (others use
stronger solvents).

The ignition switch is a problem prone area. http://volvospeed.com/bay13.htm
has replacement instructions if you have to replace it. Volvo also
has a "cmpaign" for the switch if the car won't start (not your problem,
though). In a campaign, they replace the parts for free if you complaign
about it. The campaign applies to 98s with serial number ranges:

874471590-490221 874476745-490980
875416734-435703
876416734-435703

(The campaign issue is from alldatadiy.com which lists recalls,
service bulletins and campaigns.)