I have been digging for an answer to this symptom for weeks now. I bought a 99 30th anniversary Outback wagon that does the same thing. Everything is good except 4-3 downshift which gives a flare and a slam. If I downshift manually, no problem. Tranny was flushed not long before I bought it (they had the shop receipts). It is full (I checked it).
I think I have read every post there is about the 4EAT downshifting. I have found a handful of folks describing the same symptom, but there are never any followups.
I did find a second generation transmission document from Subaru that describes the 4-3 downshift sequence. Two solenoids are involved, the line pressure solenoid and the 4-2 brake duty solenoid. That's all I have found so far that gives me any insight whatsoever and that isn't much.
I fixed my intermittent speedometer and had the battery disconnected for about an hour. When I put it back together the slam problem seemed to be much improved but it came back in full force after a day or two.
BTW, the slam only happens when I start to climb and I press the pedal to try and maintain speed. Down hill or slowing down never slams.
