Burst of acceleration when de-clutching?

Howdy,
I drive a 2004 330Ci with the M-Sport package, featuring a six speed manual transmission. I've observed something that I consider a little odd: when I
go to up shift (say, 2nd to 3rd), just as I go off the gas and begin to press on the clutch, the car will jump forward briefly. It's like the car had a sudden, short-lived burst of acceleration, though not caused by stepping on the gas pedal.
I'm at a loss to explain how or why it happens. I've never noticed it in any other manual transmission car I've driven over the years.
Any thoughts? Anybody else experience this?
Kevin
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Had a similar problem on 1987 320i. I doubt whether the problem is related though, since a 2004 would most probably have very little in common with the '87.
The problem was related to a device that sits on the throttle arm outside the air intake. It works similar to the vacuum advance on a distributor. It opens the throttle valve when vacuum drops, supposedly when the revs drop very low. The inner membrane was shot, keeping it much more open than it should have been. Idling was also a little higher than normal.
MW

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