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