I've noticed these problems with my car:

While waiting at stoplights, my car's RPMs at idle drop almost to the point where it stalls (like something just turned on in the engine). I've made sure it wasn't the A/C causing this, the fans, or anything else (they were all turned off). It just suddenly drops however many RPMs to make it feel and sound like it's about to stall. I also notice it while driving on the freeway. I'll be cruising or accelerating and it will suddenly have a dip in power. While at a stop, I'll also notice that the headlights will dim due to the reduction in RPMs. I'm not sure about the interval between occurances, but it is a continuing "problem".

A different (although possibly related) problem:

When I first got my car, I noticed that the engine makes a winding up noise when turned completely off (the keys are not in the car, nothing is turned on at all). I first noticed it while washing my car. I can't isolate where in the engine bay the sound is coming from, only that the sound is definitely noticeable standing next to or sitting in the car. It happens approx. every 10-15 minutes.

Has anyone experienced these problems with their Prelude?