1998 Subaru stalls

Hello,

We own a 1998 Subaru outback wagon. The car has 146K miles on it. We love the car and have been very faithful in taking care of it with regular oil changes and tune ups. Since last couple of weeks, the car has been stalling. It happens at random times not necessarily when it’s started cold. I would be driving at a normal 20 mph on a back road and all of a sudden the car stalls, like the gears are not shifting properly. Are there any other symptoms to look out for to confirm if it is the transmission or something else? Please advise. Any help is highly appreciated. Thanks