CNG PROBLEM IN HONDA CITY ZXI

i am having a very strange porblem in my honda city which is wen i run on CNG,car gives little jerks and den moves on,lately it started a little blasting sound too,at that moment car stops for a sec nd then moves on.and its get more and more frequent..when a approachd CNG mechanic,he told me to get my spark plugs replaced..bt i dont trust him,as wen i run on petrol,derez no such prob...well my car has run arnd 1500km on cng..total 31500km..plz advice...