Three things:

1) Try this. Start engine, let it warm up (drive it 5 miles).
Park the car, leave it running and pop the hood. Yank the oil cap off.
Does the idle get worse? Does it get more unstable and rough? It damn well better get worse, if it does not get worse, you have a bad vacuum leak.
Fix that.

2) If you changed plugs on a car with 200,000 miles and did not change plug wires, grab a reasonably large baseball bat, hit yourself with it a few times - preferably in the head.

3) If the car does get rougher when you yank the cap, I'd suspect three things: Bad plug wire(s) or fouled up MAF sensor assembly or low fuel pressure (probably not if it runs good when running down the road, low pressure would make it worse as you accelerate).