Is there any intermittent code displayed? If yes, then it seems like VSS problem. If no, you should check IAC. The most simple check of IAC valve is to slowly release the clutch in first gear. If the RPM rises to keep the engine running, then IAC valve is fine, and if the engine stalls, the IAC valve is faulty.
If the problem is only on CNG, you should service the kit and clean the cylinder from any residual oil.