How long has it been since the O2 sensors were replaced? Their performance degrades steadily from new; 60k mile replacement intervals are recommended by most manufacturers.
Factory original I'm pretty sure. The car has nearly 100,500 miles on it now? So you reccomend replacing them? What would be the best place to get them at? I don't want/need the high dollar ones, but I want to fix it right, and if the auto parts o2's aren't worth a crap I don't want them.
I'd suggest the Bosch units, and they should be replaced. Their function begins to decline from the moment they are put in. People don't like to replace them because they are expensive, but they should be viewed as a normal tune up item - like plugs, replaced every 60k or so. They'll set you back about $70-$80.
Keep in mind, everything gets worse mileage in the winter. The air is more dense which means it takes more power to push the car through it. Warm ups in cold weather use more fuel and the car runs richer when cold - more fuel. Last, the winter formulation has lighter components in it which have less energy in them - lower specific fuel density in the winter. All that points to worse mileage. Mine'll get 20 mpg around town in the summer/spring/fall -- around 18.5 mpg when it's 30F outside.