This is the most repeated topic on the FWD board....and in 98 70-class, this is an acute problem (as has been reported here by someone claiming to be Frank, a Volvo tech). It is almost certainly the module in the 98 and Volvo prescribes a swap instead of a repair. The part is $348 if you buy it yourself online - Volvo will charge you $416 for the part. The failure is rumored to be related to solder failure (Frank disputes this) and it looks like Volvo is recovering the units, perhaps for remanufacture. The new module is warranted for a year. It's located under the ABS pump block.

Please, please, please, if it's the module, do all owners of FWD Volvos a favor and report it to the NHTSA (the people in charge of safety recalls and certain TSBs) at their online site: http://www.nhtsa.dot.gov/cars/problems/ivoq/default.htm. Perhaps if they had enough data, maybe there's something they could mandate such as a recall, or at least a notification to all owners of 98 70-class Volvos.