haha......brother this fault is also FF in Suzuki CNG cars... My EFI FF CNG Cultus 2007 also does the same, and once i was driving with 3 lights green showing CNG and all of a sudden car started jerking and i noticed that there was no CNG in the tank :P. Luckily, i keep half tank of petrol all the time in my car for emergency use !
on the other hand, my brother's Khyber 2000 with aftermarket CNG kit has never given any trouble by any means till yet MashaAllah
so the problem seems to be in the CNG switch/sensor, just get it changed, not a big deal !