Auto acceleration & white smoke with engine stopping

I have peugeot-206 1999 manual driving model, it has 191k mileage. It runs very smoothly when I am driving with very minimum smoke but when I stop at the red light even on neutral gear, it increases the RPM even I am not pressing the accelerator, it automatically accelerates also when on gears. When it is stopped at red light, it leaves lots of white smoke & sometime stops the engine & takes 6-5 key turning or 2-3 mins stuck on the main road.
please suggest what to do & how much will this cost? I want to use this car for another 6 months due to re-location to a different country.