Air Turulence!

I'm not sure if this is for this plane or most but here it is.
1. In a dassault Falcon 90VB, when it reaches a certain speed/height in just shaking up and down violently. :-/ It doesn't crash it, it just takes away from the scenery. Any suggestions?
2. In gulfstreams, theres one that is about 15 mb, found at Surclaro. It looks good and all, but when you get to a certain speed, the tail just flips out and sends you in a downward spiral...
Suggestions yet again...
Thx
1. In a dassault Falcon 90VB, when it reaches a certain speed/height in just shaking up and down violently. :-/ It doesn't crash it, it just takes away from the scenery. Any suggestions?
2. In gulfstreams, theres one that is about 15 mb, found at Surclaro. It looks good and all, but when you get to a certain speed, the tail just flips out and sends you in a downward spiral...

Thx