Jolting vibration coming from engine

I have a 2000 V6 4WD SE, while idling there is a sudden jolt/shaking vibration that comes from the engine. Its not a violent jolt but you can definitely feel it. It happens about every 15-30 seconds or so. Someone told me it might be a belt slipping. I changed the belts and it still does. Any ideas on what this may be?

Doubt it's a belt.

Could be a flaky plug, plug wire, cap/rotor, or even a failing distributor.