Sounds and Feels like it's missing?

I have a 2003 2.0 with 23K miles on it. For the past two weeks or so I have started to notice a little engine vibration at idle, and my gas mileage is
going down a bit. :-( Since the New Year temps here have dropped to around 32 degrees F.(0 celsius)
In my blazer, this always meant Spark Plugs-about every 12K miles. How many miles are the Focus Plugs good for? I have no maintenance record as I bought this car used.
Some of the other posters are talking about a DPFE sensor?(the guy whose engine was missing 3 times a second), would this be the culprit if spark plugs are supposed to last longer?
Could it be temp related? My Blazer alway ran rougher in the winter. Thanks. -- Dan 2003 2.0L ZX5 23K Miles Sagria Red VA - USA "Those who forget history are condemned to repeat it" - George Santyana
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