This can be a problem of your tunning, its CNG is not well adjusted, because of which engine is getting some extra load..... if the pick is perfect then go for its AC gas pressure...... open the bonut, there is a steel bottle there, it has a glass at top of it. when you switch your AC on you can see the gas flowing through it.... this gas is white in color.... if the gas is not flowing good, means you dont see good flow but little gaps in flow then the pressure is low, go for the gas refil.... whenever you refil the gas say him to open up the condenser inside car and clean it too..... further..... you asked for a mechanic....... i live in rwp, when i had mehran i got its AC serviced from KALI TANKY, near chandni chowk, that time i got satisfactory work, before this i changed AC mechanic 2 times. i hope that there is no heatin problem in your car, if there is then first let its radiator cleaned.