2005 Subaru Outback 3.0 r begins to miss at higher speeds, gets worse when you decelerate. Any ideas?

Most of the time that's caused by a tiny crack in the ignition coil pack //// an old trick to determine the problem is ... wait until it's dark ... raise the hood with the car in park have a friend start the engine ... walk around to the front and look at the plug wires and the ignition pack ... as the engine speed is increased you will see the arcing/sparking become visible in the dark.... for safety's sake ,, keep your hands in your pocket and not inside the hood

A new set of spark plugs might also clear the problem up.

Check MAF sensor or wet ignition wire (was it raining out when noticed?).