As title says. It toggles on and off after doing /nv 
Help please.

Do you have any plugins installed that modify the time or weather? These types of plugins can cause conflicts with NightVision. One plugin that comes to mind is PreferredEnvironment.

Yes, tried unloading both PreferredEnvironment and RealisticWeather (one at a time) and it still kept flickering on and off.
But it solved itself after wiping the server and map.