low fps with vsync on

hi all, i have P3D v4 and 737 NGX installed and have found that when i try to run NGX with vsync ON  (the low refresh rate & unlimited fps trick for smoothness/tearing)

i only get 5 frames per second. turn vsync OFF and i get 40-50 fps but lose the smoothness. all other aircraft work fine with vsync on. i use nvidia 3 monitor surround for outside view plus panels on a 4th.

any ideas/suggestions greatly appreciated.

pete.