I have a 2005 Impreza 2.5 RS with a 4EAT transmission. It was good for the first 4 or 5 years, but then shifted rough when cold 1-2 and especially 3-4. It gradually got worse, to the point where I kept it in 2nd gear until the transmission was hot. In spite of many trips to the dealer, their only solution was to replace the transmission. Recently, my car wouldn't idle consistently and the dealer cleaned the throttle body and did a re-learn on the idle. Besides fixing the idle, cleaning the throttle body made the transmission work nearly perfectly, even when cold. Does anyone know why cleaning the throttle body would improve the operation of the transmission so dramatically? Is there some vacuum feed to the transmission? I though it was all hydraulic.

any other work done at the same time? maybe some ground connections were refreshed?

If they disconnected the battery, that wipes the transmission learning.

 

My H6 gets harsh shifts and a nasty jolt on the 4-3 kickdown after awhile, disconnect the battery and all is good again.