Misfires in Subaru's are more often fuel related rather than spark. That being said, having rebuilt and work on a few (higher mileage) '02s lately which I notice we all have, another common and possible cause is valve lash being tight. As the seats wear, the clearance between the cam lobe and bucket shim gets smaller than spec. Eventually this causes misfires due to the valves opening an RCH too early.
After exhausting the possibilities of vacuum leaks, clogged or leaky injectors, tired plugs or dead coil packs, that is really the next thing to investigate. Subaru actually recommends a valve adjustment for our engines (I forget the suggested mileage). Unfortunately that is an operation that most shops can't even perform, and it's quite costly.
BTW, BG makes a good injector solvent.