Multiplayer jumping planes

Hi there...I hope someone's able to help me with this problem, I usually fly at nights with a friend via multiplayer session (IP direct connection), and normally there's no problem, but a couple of weeks ago, I had to reinstall the FS9, and now... when we are flying, the planes keep jumping from side to side... that is... they dissapear and reappear constantly... It's very frustrating, because we cant' fly in formation, cuz some times the plane jumps right on top of the other one and it crashes...Hope someone has some experience with this kidn of thing.Thanks,Pedro