What is the square metal box under the air filter assembly, you plug a 4-pin round connector in it?

Tonight my turbo did something strange. I noticed the car did not have any power in upper RPM's, so I floor it, and there is no power from the turbo, but I hear a faint whistling noise, then at around 4,000RPM it finally wakes up a little.

Now the turbo appears to be acting normally, but it is getting a little intermittant. Just added another qt of oil tonight to make 3qts in 3150 miles, god damnit.