"Decreased Engine Performance" readout
 

Car is a 2006 V50 T5 AWD, only 78,000 miles, well maintained, conservatively driven.

Every couple of weeks recently, i've been getting a readout in the message center, "Decreased Engine Performance." No check-engine light, no other indications of anything awry. (It's a message I can't even find referenced in the owner's manual.)

Typically, I erase it and ignore it, since the car feels totally normal. I usually accelerate "normally" to 70 or so at 2,150 rpm in fifth and cruise on highways, and the car feels no different.

This morning, however, I was leaving a tollbooth array and found five crazed New Jersey drivers behind me, all desperate to be first into the two-lane highway. So I punched it to get out of their way. The car shifted down from fifth to third==maybe it was from fourth to third at that point--and I found myself doing 4,100 rpm, full throttle.

If anything, the car slowed rather than accelerating, and I suddenly discovered "Decreased Engine Performance." When I tried it later on an empty stretch of road, I found that the instantaneous mpg readout showed a decreased fuel flow in third at 4,100 (about 33 mpg) versus fifth at 2,150 (typically high-20s mpg instantaneous fuel readout).

Does anybody have any idea what's going on?