Seems that these cars like "Volvo" spark plugs. Yes, I know Volvo does not make spark plugs, but it has proven difficult finding who the original manufacturer is. Actually, I just picked up a set from Tasca for around 24 bucks.

Also check the plug wires. If they're the originals, they may be due for a change. It gets pretty hot under there.

Then I'd say the usual suspects are dirty MAF, IAC and throttle body.

Yes, they do run very smooth. Unfortunately lately ours will sometimes stall immediately after start-up. Happens maybe once every two weeks. It's just strange in that in the 4-5 years that we've owned it, that never happened. Ours has 175K on it.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig