Focus Surging Problems

I have a 2001 Ford Focus SE (4 cyl) with about 52000 miles on it. A new problem has popped up in the past few days. Sometimes, when I am stopped at a traffic light, or in park with the car running, the car
will start surging, as in the engine will pulse. If I let off the brake and don't give it any gas, it will surge forward slowly. If I give the car gas and drive at normal speeds though, there is no problem. Tonight I noticed that the lights dim and come back while it is surging (almost like it gets close to stalling and then picking up again... when it is low, the lights dim). I also hear a hissing when I open the hood, but I'm not sure now if that's normal for the engine or not... I'm worried it isn't. The engine temperature seems to stay normal no matter how long then engine has been running. I haven't had an oil change in a long while, I'm going to get it changed tomorrow to make sure its not bad oil causing things to stop up, although I would think if it was bad oil that it would affect things all the time rather than just at low idling.
I also have the new fuel module (from the recall), so that's not an issue.
Any ideas?
Chris
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I guess you have a vacuum leak or a stuck PCV valve. The fact that the car runs normally at speed gives that away. First try to pinpoint the source of the hissing and see if you can spot a broken or detached vacuum connection. If not, then the next thing is to run the computer for codes, which requires a shop with a diagnostic tool. Probably easy and cheap to fix, but you have to pinpoint it first.
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Chris, I have the exact same car and I am having the same problems except for the hissing. I have my oil changed every 3 months and this problem still occurs. my engine light also stays on. I had it checked and the garage said that I had my gas cap too lose. he fixed the light problem for a brief moment and 2 day later....it came back on and now my car surges more than ever. If you find the problem...please share...:)
Amber
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