Guys I am not sure how the model flight characteristics are put together in FSX or flight sim models but if these factors roughly equate real aircraft then a dutch roll is possible or oscillatory instability.

In the real world the game is about minimising drag (pushing the airframe through the air dependent on wing shape and aircraft shape) and maximising thrust from the engines. The four basic components of why an aircraft fly is that lift is sufficient to equal weight and thrust will equal drag. Drag is very high at low altitudes but thrust is also very high, at high altitudes drag diminishes and thrust diminishes but is a curve and the outcome is your fly as high as you can for maximum range, in general terms for jets that is between about FL300 and FL420. This generally referred to as the 'best economy band'for a jet airplane. The basic aerodynamics of jets at high altitudes v pistons at low altitudes are simple you encounter reduced aerodynamic damping, reduced stability, restricted operating speed range and reduced manouevrability, in short the margin between a stall and flying ok is small. There is then the yaw-roll couple in aerodynamics. If you roll the aricraft will then yaw or if you yaw then the aircraft will then roll. This divergence backwards and forward of rolling and yawing is the dutch roll phenomenon or the rolling and rocking described in the intial posts. The ability of the aircraft to dampen out these deviations decreases as you get closer to the stall speeds so if your at very high altititude then such behaviour is to be expected especially from a swept wing design such as the 707.

AS for the performance stuff, simple to remember, if your at Cairns Australia and the OAT is 35C then it is ISA +20 and if your at Seattle Washington at the OAT is 5C then it is ISA -10. The charts just take the complicated math away from working out what is the best altitude I can get to to to have the fastest speed and minimum fuel burn.

Hope this helps a little.