Originally Posted by bjkroll
Since these are electronic controlled, I can't see a vacuum leak causing any type of slip. But that's just my opinion.
I do know from experience that they can behave very strange w/o the right amount of fluid. Try a good ol' fashion drain and flush, with the proper fluid. Another thing to look into is if the transmission pan has any type of dent in it. A poor mechanic would use the transmission pan as a way to support the transmission when pulling an engine. Putting even a slight ding in it can block the pump, possibly causing a slip.
The pump is behind the torque converter; however, a dent or damage to the trans pan could injure the valve body, which can cause poor shifts and slippage.
Most Subaru ATs work better slightly under full; they start to have issues when they're overfilled.