Jettesen... As has been mentioned earlier, that's quite normal. To level off at a particular altitude on climbout the engine power is reduced. Equally, on descent the engine power is reduced until the a/c approaches its cleared level and then the power is increased. In a busy environment such happenings will be quite common. Eg climbing out of Heathrow it's common to be stopped at 6000 ft... then a few minutes later receive climb to a slightly higher level... then a few mins later a highe level... etc, etc. Watching the dear old Conc on noise abatement used to be fun - it would rocket upwards with full re-heat, then the torch would go out and the thing would appear to drop like a brick! In fact it was just the noticeable change in rate of climb.