I had the a similar issue with my 98. The damn thing would eat gas and it had an awkward idle (would be start way high, drop to ~500-600 rpm). I tried alot of things. And It ended up 6 months later (I only drive about 4K a year) The O2 sensor completely crapped on my way back from school.
So your case may be a dieing 02
But lets not forget other reasons why:
-Dirty MAF Sensor
-EGR Clogged
-EGR system disfuctional (Broken/bad vaccuum line)
-DPFE sensor failing
-plugs? Last time you changed them?
-Air filter, not dirty or from ebay...
-small air leaks make lean codes, but will fudge with rest of computer controlled systems
-Summer time now, are you using A/C?
-Are you hauling around any extra weight? (My father forgot he had 2 x 32 count pack of bottled water in the trunk for a little over 2 weeks)
Thats probably best place to start. Good luck
