This problem has been reported for 98s in the past (you could probably
search the arvives and find a few posts). There was a recall
for some cars. People have reported (here) they have the problem even
though their calls are not on the recall list. So, I think you need
a new ignition switch. Recall info:

A. Service Campaign 83D Description

Volvo Cars of North America, Inc. and Volvo Canada Ltd have announced a service campaign affecting 1998 Volvo S70/V70 models. Approximately 5,831 vehicles in the US and 1,035 vehicles in Canada are affected.

The ignition starter switch should be examined for proper operation. An insufficient electrical contact during cranking may result in an open circuit in the power supply to the starter motor and the vehicle may not start. In affected vehicles the ignition switch will be replaced.

All owners of these affected vehicles will be notified by mail the week of June 1, 1998

B. Vehicles Involved

Assy plant 1 VTM
874471590-490221

Assy plant 2 - VEC
874476745-490980
875416734-435703
876416734-435703