Problem with 2002 Kia Optima?

[Sarah]

I recently bought a Kia from the ford dealership as a drive car because I want to fix up my older car. Ever since I got it, it has been having a problem. Sometimes when I stop at stoplights, switch gears, or even when I am driving the car will just shut itself off. Ill be drinving ans the tachometer just divebombs sometimes even a couple times in a row. I have brought it in to the dealership (theyve offered to fix it for free) twice and it they say it never happens to them and they keep it for days, and never find it. Anyone know what it may be?
And I cant buy a new car, I just bought this one.