Can you figure this one out? I drive a 2002 Ford Taurus that suffers from a disease with episodic symptoms. Most of the time, the vehicle operates normally. Occasionally, the car will exhibit vibrations of unequal duration at idle. The vibrations that I can feel are probably between 1-4 Hz, and they exhibit no discernible pattern. When this occurs, the engine idle is also uneven. When the vibrations more intense, the engine sounds like it will stall, but it never does. When I step on the gas pedal during one of these episodes, the acceleration is jerky and slow, but still responsive. When my speed is sufficient for the transmission to go into second gear, the symptoms disappear. That is to say, as far as I can tell, the symptoms go away. But when I stop at a light, the jerky idle symptoms return. Often when this happens, the check engine light becomes illuminated. By habituation I have learned to ignore this light. If I drive for long enough at highway speeds and then come to a stop 20-30 minutes later, the symptoms are usually no longer present at idle anymore. Yesterday, I pulled into a parking garage having driven about 2 miles in city traffic and having experienced no symptoms. When I got back to my vehicle and started it again, 20 minutes later, the jerky idle symptoms were worse than they had ever been. This time, pushing the accelerator had close to no effect until I floored it. Only then, with much hesitation, could my vehicle make it up the ramp and onto the street. After driving for 15 minutes or so, the symptoms were no longer present. My question is, what’s causing the problem? Is it a timing belt issue? If so, why do the symptoms come and go? Could it be carbonized debris accumulating under the head gasket, occasionally breaking off, and causing detonation? There are clearly many, many possibilities. But which one is the most likely? And, should I see a mechanic soon, or should I just ignore it until I am forced to deal with it? Thank you in advance!