the A/C idle up is controlled by the ECU, and its done by the ISC motor, for it to work smoothly it should be lubricated properly AND - the idle speed set correctly first, CNG mechanics disturb the idle stop setting causing misinformation to be relayed to the ECU which then causes such problems.
Correctly adjust the throttle like the factory wants - then work out the problems, if the ISC is stuck or bad - the CEL will be on with its code.