what kind of plugs did you put in? check them again?

you could check the o2, iat, and map sensor signals.. those could be off
but only the o2 would have potential to adjust back with a plug change, but going bad again that quickly? not sure about that..

How many miles were on the old plugs? were the 4 uniform in their condition? did you replace the wires or check the coil pack?

if you had that terrible of a ignition system before the change you could also have a clogged cat, or partially clogged. That would restrict your mileage, an with new plugs you would have been able to temporality burn out some of the cat to show improvement (no scientific data to back it up, but the heat from the properly burning cylinder transfers to the cat to allow it to function, a properly running engine with semi clogged cat would could get hot enough inside the cat to burn off some restriction). Theory, a fully clogged cat on some cars can bend valves, i replaced an engine or two because of that.