Well, I've tried the "Diamant" model with its original .par file using the standard "World" landscape file and it seems fine to me.
Have you modified the .par file?
What settings are you using for Wind and Thermals?
You can find the exact numbers within the Windows Registry under "My Computer\KHEY_CURRENT_USER\Software\Flying-Model-Simulator\ThermalForm". Use REGEDIT.EXE at your own risk.
My settings are BubbleZ = "2" and iBubbles = "40".
If you turn off wind and thermals and still have a problem with models, then it's a problem with the model.
Several of the models on my web site are a bit twitchy, just like the real models. Many times I do not have the actual models myself and must rely upon feedback from actual owners to tweak the .par file. Feel free to make any model more stable by editing the .par file yourself. I like Mr. Masuoka's paredit.exe program located:
http://rcp.web.infoseek.co.jp/fms_e.htm ... _Converter
You may want to uninstall FMS and reinstall it to clear any issues that may have cropped up in the Windows Registry. It's helped some other people get back to the base install.
-- ggunners