2006 Chevrolet Optra start and stop , self problem

my chevy optra 2006 have self and start problem I have to do couple of selfs until its starts, I got engine spark plugs replaced but at signals where i have to apply breaks the speed of the car and power drops and its goes 40 km /h on the speedo meter, if i go gentely on break its some how ok but the problem presists, its can get sloe at any time and jerks
Thanks