Strange problem

Hi. I have an 2000 Focus with a strange problem.

When the engine is idling, the rpm's will dip slightly and the engine will
shake (I can feel the vibration when sitting in the driver's seat) as if
it's going to shut down, but then it just resumes idling normally. That
behavior repeats once a minute or two (more or less). The symptoms are more
noticeable when the air conditioning is on, but they do occur when the A/C
is off as well (although, they're less pronounced then).

I've already replaced the spark plugs and the plug cables. (No effect.) I'm
also planning to replace the fuel filter. Does anyone have any ideas or
suggestions?