Having problems starting,and when started feels like its going to stall,feels juddery at times when driving

i am have problems with starting car.this is intermittant and is unreliable.when i get it started it feels like it is going to stall especially when sat at a junction or lights.The car has cut out on me twice now at a junction.I have had 2 mechanics look at it and cant find the problem.At times when driving it feels like I have kangaroo juice in the tank.I hope you can maybe give me some idea as to what the problem could be.Many thanks June