[Bassasasin]

O2 sensor is the most often culprit.
Air intake filter. Of course theres the usual things like missing and the like.
There are things that some do to help like facing the plugs.

However not from your issues of one day to the next or after a repair job. I hope you havent got a new vacuum leak.

21 Mpg in the city is great.. actually 27 highway only is whats been experienced.

I have heard of lower mileages when running Ethanol too since it has a lower energy available when it burns.

Good Luck