I have a '97 Outback that came with fobs and a dealer installed Subaru alarm. It started flaking out on me about 4 years ago. First I removed the actual siren - that helped the unwanted 2 am wakeup calls, but I still had trouble with the car not starting, usually when the weather was cold.
You know about the override button under the left side of the dash? That will let you start the car at least.
Your '98 may be different than mine, but on my '97 there's a black square box under the dash that the dealer ziptied to the left of the steering column. On mine, you can open that box and there's a bunch of dip switches that control various functions of the alarm.
I don't remember now which switch was flipped - it was a bit confusing and I had a guy help me.
Fortunately, there's a web site that has the manual online plus some tips:
Russ's Little Subie Page (click on Keyless/Alarm Project)