My 2001 Ford Focus judders when idle, and is most seen under load. The rev counter goes from roughly 500rpm to 1500rpm during this juddering cycle, as if it is "hunting" for the correct place to be.
When the lights are on, they noticeably dim during this time - which lasts for about 2 seconds, every 15 to 20 seconds.
The alternator is the likely culprit, however this was replaced two years ago and the battery is still being charged.
This behavior is most noticeable under electrical load, but also happens when the lights are off. This behavior is intermittent, and does not occur when the car is in motion - only when idling.
So, is the alternator the culprit, or is there something else which may be causing this?