Engine Stop when press clutch in shifting down gear, also RPM fluctuate in idle mode

Since last 2 week, I m Facing Problem in driving car. When I press clutch, engine stop down, in shift gear down condition, this problems occurs both in CNG and Petrol Fuel. This problems occurs sometimes. Also RPM fluctuate in idle mode some times 500 to 1500 range this problems show in Hyundai workshop and also local garage. According to them over all service done for my car, throttle body cleaning, change Spark plugs, cng kit service, error correction, RPM Set, but didn't solve problems yet. So please instruct me what is the reason for it. and how to solve this issue.