The cylinder head usually has a life of 150,000 km. Have you done that much?
What I think is the accelerator wire has increased in length (khichao) due to continuous use. Now it can't maintain the proper tension when idling. Why don't you first simply get the lever adjusted which the accelerator pulls?
Or the mixture?
The other thing is: Maybe the air filter is choking up, so the engine can breathe properly at idle, and it dies. (Remember engine sucks air like a pump, so the faster its boing the more powerful its suction will be).