Here are a couple of thoughts for you to check out. First, take a look at the transmission fluid level. If it is down a bit, say half-a-quart, it might be just enough to change the internal pressure so that it would take longer to shift into gear. After it does go into gear, it is likely the temperature has come up enough and the transmission fluid has expanded enough so that vanes that might not have been covered by fluid are now covered the transmission works properly.
To check the fluid, park the Outback on a flat surface; set the parking brake and put it into park (you set the brake as a precaution). Then, let the Outback idle for about 20 minutes to bring it up to temperature and then, with your foot on the brake because you are taking out of Park, run the Outback through each gear so that fluid circulates throughout the transmission. Then, after you put it back into Park, pull out the transmission dipstick and take a reading between the HOT settings. The chances are very good that you will find the transmission down a bit. Why would this happen? Over time, transmission fluid does heat up and burn off a bit because it is primarily an organic. So, it is quite normal for transmission fluid to drop a bit.
To refill, obtain the fluid recommended by the Outback’s manufacturer and refill it only until you reach the top line on the HOT setting.
The other possibility is that the torque converter may be having a problem in that it might not be generating enough force to cause the transmission to slip into gear. If this is the case, then have a trained technician take a look at it and see if the torque converter cannot be repaired as it sounds like the rest of the transmission is in good shape.
One other possibility, though it is a long shot, is that one of the solenoids (there are usually two) that actually set up each specific gear may be slightly lazy. If this is the case, then it should also be replaced.
All told, the repairs should cost anywhere between $160 (solenoid) to $700 for the torque converter.