I'm trying to pilot a POSKY B-767. It performs quite well on take-off and seems to climb ok. However, it appears I can only operate it cruising around FL180. Anything above that I seem to have problems maintaining speed above 210knt indicated speed.

All the other stock a/c and other plane's i've downloaded are unaffected, apart from my vistalines 737-500 which appears to be similarly affected.

Any help appreciated.