Thanks everyone for all the prior help. I've got another one for you, unfortunately. Here goes...

Sitting in park, engine idles smooth, but there's a slight vibration at all RPM rates except for about 1,200 (vibration's much worse here) and 1,800 (it smooths out nicely here). There are no warning lights, and yes they all work.

When viewing the engine while it's running, it stays very solid, with very little movement (helping to eliminate mounts).

The service history shows that a coil was replaced, then ten days later it was replaced again due to the replacement being defective. Perhaps the replacement's giving up again? It's been about a year since it was replaced.

It feels like the vibration is once per engine revolution. What list of items should I knock off to get this thing back to normal? Assume only "by-the-book" maintenance has been done on this 155,000 mile car. Thanks in advance!!

 

I doubt the coil is failing. If it was, you would be getting misfire codes.

How does the engine run under load when driving?

 

It runs well enough under load, though I don't load it heavily, just normal driving. Accelerating up to speed I can feel the rough spot at 1200ish (it moves around a little with temp changes), but past the rough spot you wouldn't even know there's an issue. That said, one time I gave it a bit of gas to get the transmission to downshift and it hesitated briefly.

 

Well, it finally threw some codes and triggered the engine light.

P0353-Ignition Coil C-Primary/Secondary Cicuit.

P0430-Cat System Below Threshhold, Bank #2

P1000-Generic.

Hopefully replacing the coil will also kill the other codes. Now I just have to figure out which coil is C.

Needless to say, the car isn't being driven till this is resolved.