I would say this, had the same problem with my 99 Forester. The solution is NOT a transmission flush alone. There is an external screw on paper media filter(like a motor oil one) on the drivers side of the transmission that needs to be changed. I took mine in for the antifreeze recall and had them flush the trans, as I was experiencing a few seconds lag in it engaging drive. It was just over 30K at the time and due. They flushed it and it was the same afterward. I then did some research and found out about this filter. I changed it and have no issues since. If you get a flush make SURE they change this filter, the flush won't touch it.
