What can cause 2005 Volvo V-70 stumbles and misses when idling?

Car runs good- gets good mileage 24/29, but will skip and miss when at idle.

Its been in the shop a couple times for this problem- and it seems to do okay when I get it back..... But within a few hundred miles- it starts again.

Any ideas appreciated.
Update: Gentlemen- thanks for the answers. I'll look into this OBD thing.
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