I had an almost identical situation on a 97 Outback with 2.2 swap.
 
	Ended up being the O2 Sensor.  
 
	I can't find any info from Subaru about how the O2 affects ECU's timing calculations.  But in this one case, for me, the timing was randomly retarding to less 13~15 degrees under light throttle cruising (when it should be more like 20~35)
 
	When the timing would pull back, the car would feel sluggish, no throttle response.  Although for this case it was not a bucking, jerking loss of power.  More like when you get to a slight hill and try to give a little more throttle, nothing would happen for several seconds (timing at ~15)  then would kick in (timing jumps back up to 30 or so) and then acceleration would feel normal again.
 
	Putting in a brand new Denso O2 sensor seems to have stopped that issue and car drives normal now.  Timing stays where it should.