I've noticed this on all the Tu154 videos I've watched, and when I flew on one recently was waiting for, and heard/felt the exact same thing.
When the aircraft starts its takeoff roll, after a few seconds the thrust reduces temporarily, then rises again. I've watched flight deck videos and never been able to see what it is that causes the engines to throttle back, and why it's done.
You can hear it quite clearly in this video (takeoff is at the 22 minute mark):
Can anyone shed any light as to what this is?