Hello Gustavo,
This behavior is normal in some situations. The first thing to do is to wait. In some cases, the Scenery Generator can take up to one+ minute to settle, and load the full scenery. During this time you will see very odd things, like the aircraft below the ground, bouncing, etc... this is normal.
In some (rare) ground loading situations, the PSX aircraft keeps bouncing, and even (in very rare cases) crashes. The ONLY thing you have to do is to reload the PSX SITU. This will force WidePSX to reset the synchronization process and everything will come back to normal. For this purpose, the good habit is to always save the SITU (or load from a saved SITU) in PSX before to connect WidePSX.
Best regards,
Jean-Philippe