@bubble get your CNG switch checked. I think it must be a wiring issue as you mentioned installing a new CNG kit.
For any CNG Car, you convert it to petrol directly w/o keeping it in neutral stage of the switch - The engine must shut off in moving from CNG to neutral as supply of CNG would be cut immediately - It is only when converting a Carborator car from petrol to CNG, you need to consume the petrol in carborator using the neutral stage of the fuel conversion switch.