Sorry to pull this thread from the dead but this is the exact same problem I am having with my girlfriend's 2007 legacy sedan (non-gt) and have found no information anywhere else. The problem is intermittent since she bought the car ~3 years ago but seems to be happening more frequently over the past month. Note: The MPG Err message is constant, no matter if the cluster is working properly or not.
In addition to this problem there are a few other things going on, just as an FYI: No cruise control (never worked, replaced clockspring, didn't change), occasional starting issues in damp/wet sometimes but not always (suspect coil pack).
Recent work: map light assembly to fix airbag light, airbag recall ~1 year ago, cat + o2 sensors ~3 months ago (did not alter this problem one bit), plugs wires battery ~1.5 years ago. None of these had any effect on cruise control, MPG display, or occasional cluster issues.
Top: Blinking "- - - - -". Odometer and trip meter still function (if you zero trip it will record the number of miles and display properly after you restart the car)
Middle: MPG display "Err" shows up on MPG average and miles to empty as well
Bottom: Gauges don't move while driving, gas is at about 1/2 tank but reserve light shows, blank odo/trip
http://bwindle.com/image/Issues001.jpg
coblue, did you ever find a solution?