2003 Mercury Sable shudders when idling/shuts off
 

Hey, all! My Sable's check engine light has come on and it's been giving me some rough idling. Also, the tachometer will steadily rise while I'm accelerating, usually up to 400 RPM or more, and then abruptly drop when I take my foot off the accelerator, down to about 300 RPM.

When I'm stopped at a light or a sign, the car will shudder and the tachometer will freak out, wobbling around 1 to 300 RPM. Occasionally the car will just shut off, and I have to put it in park and turn the car back on. It's worse when I turn the AC on.

I suspect I might need to replace my spark plugs, but it could also be a problem with the catalytic converter; I'm just not sure. Any help would be greatly appreciated!

First thing's first, I'd take it to Autozone or other store that offers free code reading. Knowing what that code is should help a little at least.