It is most likely that the mechanic fiddled with the idle adjustment bolt located on the throttle body, while the engine was running on CNG. Get the the idle RPM adjusted while on petrol at about 800. It should be done by a professional, since I think you might mess-up the situtation. Then turn the engine to CNG and try to adjust the idle on the CNG kit. Hopefully your problem will be resolved. It is also worth niticing that the A/C won't work if the engine RPMs go below certain point like ~500RPM....
Another important thing is, if you have installed the advancer and the mechanic forgot to connect the advancer return wire, the engine RPM on Petrol also increases.....get the connections checked!
You may contact the following person to properly address your problem:
I-10 CNG station, Opposite Suzuki Azim Motors