Yesterday I took my brother's Cultus VXRi (2010) 12K driven to Islamabad via Motorway. I noticed a strange behaviour when driving at 120+ kmph on CNG. The car would occasionally take a jerk, like the engine stalling for a split second and then everything becoming normal. The frequency increased with the passage of time (I drove non stop from Lahore to Islamabad and faced this behaviour about 10 times in the entire journey). On petrol, this behaviour was not observed even at speeds in excess of 140 kmph.
When I asked my brother, he said it was "normal" behaviour, but I do not buy it.
Anyone can shed some light on it?