My 2002 Ford Focus does this horrible jerking motion when I drive?

So when I am driving my car does this horrible jerking motion. Sometimes it even shuts off when I'm idling. When it shuts off, the electric system is still working (i.e the radio, hazard lights, etc), but all the lights come on such as engine, battery, etc. As soon as it comes back on all the lights go off as if nothing happened. I took it to Firestone and the mechanic guy rode in the car with me around the block and we could no replicate what had just happened. They checked the car out and the only thing they said need replacing was the plugs so I got them replaced. It still jerks worse now though. I recently got a belt and pulley replace so it isn't that and it isn't any hoses either. I'm thinking about just getting another car because this one is turning into a money pit. It's only got 115,000 miles on it and I have regular maintenance done for upkeep. Any suggestions? Oh and it's an automatic,

Read more: My 2002 Ford Focus does this horrible jerking motion when I drive?