annoying problem

** 98 GS ** This past weekend i went to a tuner shop and they pulled the code for me. Happened to be the 02 sensor.. got that fixed and everything. A day later my check engine light went back on. some of the same symptoms still occur. like whenever i use my breaks.. when it gets to a complete stop the rpms go all the way down to 600 then goes back up to 900. The check engine light is very dim... almost looks like it doesnt want to come on. I wish it didnt lol... im really stumped. The engine itself only has 9 thousand miles. Pulled from a junkyard. everything else seems to be working perfectly... Help anyone please