Is it the first flight after you installed this scenery? Normally this is typcally an conflict, in my experience in most cases with textures or AI. The only advise I can offer is to uninstall. Check whether the problem is gone without the scenery and re-install the scenery again.
If you can get a fresh download, install this fresh download.
If this doesn't work you can try to give the scenery a higher or lower priority in the scenery list.
I hope this will work for you,
Cheers,
Huub