03 Focus "sputters" when accelerating... need help

I have a 2003 Ford Focus with 42500 miles on it. Recently my car began to act rather oddly... when I slow down or come to a complete stop and then press down on the gas, the car begins to sputter like
as if there is pockets of air in the fuel. The car does accelerate but it takes the car some time. There has been several times when I turn on the car and the air conditioner and the car begins to shake. No warning lights come on when these things do occur. On July 1st I took my car into the dealership for the 42000 mile check up and everything was concluded to par. This problem was first noticed yesterday on the fifth. What can be the problem? I have scheduled to take the car back into the dealership on Friday of this week, but it would be so lovely if I could figure this out before then. I hope that someone out there may know what the problem may be.
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Could be fuel pump but this is not quite the normal symptoms. It could simply need a tune-up i.e. plugs and wires. You could also try a can of injector cleaner, but this sounds a little sudden and extreme for that.
Might also be a vacuum leak, possibly from a split hose, or a malfunctioning EGR. One simple check: leave it idling, open the hood. If you hear a hiss, it's vacuum. One final thing, you could have gotten a tank of bad gas.
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