Cadillac Repair: 95 Eldorado, gas mileage, battery connections

Question
I have a 95 Eldorado 4.6 northstar about a month ago it started missing and backfiring, it would only backfire when I was increasing the rpm's from a stop. I took it to a qualified Cadillac mechanic he repaired it now it started doing the same thing all over again it feels like it's surging really bad when at a stop. The rpm's dropped from 1100-500 when it's surging the gas mileage is bad. And while I'm at it when I'm driving at night you can look at my dash lights interior lights and headlights they are surging also kind of like it has a heartbeat but the alternator is in good working condition the car has 130000 miles on it.   I'M STUMPED

Answer
I was thinking the alternator as i was reading it until you said the alternator is good. Check the battery connections and if you have one available test the voltage at the battery while the car is running. Check your belt and then, remove it, and run the engine, dont drive the car cause your a/c(MAKE SURE THIS IS OFF!!!) power steering and any belt driven accessory will NOT work. Dont run it for more than 5 moinutes however, there is no need to. Let me know if the problems eliminated with the belt off.