I took a 737 on a hop from KATL to JFK today and listen to this...

I plugged in FL330 and clear weather with no wind.  Fuel was 40% in each wing and 50% in the center tank.
At about FL180, the aircraft started a small, but noticible "rocking" motion.  I rechecked the panel to see if fuel was accidentally transferring from tank to tank, but it was not.  Rate of climb was 2,500 fpm and as I climbed, the rocking became more severe.  By the time I reached FL330 the rocking was almost 10 degrees from side to side!

Any idea what that was all about?  Anyone ever experienced something similar?

Stingray686,

I've encountered a similar phenomenon with add-on aircraft where the plane will reach cruise altitude and begin "swimming" through the air like a dolphin.  My solution to this has been to substitute the .air file of another aircraft that works (perhaps a default aircraft) into the folder of the aircraft I'm trying to fly.  This has fixed the issue of level flight, but I haven't encountered your specific problem, so I don't know if it would help you or not.

A more pertinent question would be if you are using an add-on aircraft or not!?! ???

Erik