1987 firebird 2.8 v6.. bipolar car.

I bought this car in august. i love it, but around christmas it started acting up. sometimes it will run amazing, better than the day it came out of the factory. other days it wont even start. had a mechanic do a major tune-up on christmas day, ran great until now. yesterday it wouldnt start. the engine will turn over, but not start. im thinking a fuel problem, as the fuel filter probably hasnt been changed for a long time.
has a new battery, new spark plugs, new distributer, added a fuel system cleaner around new years because he said the injectors were filthy. seemed to work fine up until yesterday.
the catalytic converter is pretty much in pieces, obviously need a new one. but i dont think an exhaust issue would cause it to run like this.
as of right now, the car wont start. its freezing here so its plugged in and that seems to be helping.
any ideas?? fuel filter is next, but what else could cause this?