Car back fires and stumbles?

It's a 97 monte carlo 3.1 runs great if you can get it to start and get it in 2nd gear then it runs like it always has.Just started it yesterday had it hooked up to a obdII machine and all it showed was a random misfire(already knew this) being that every thing is electronic on the motor and every thing runs off something else which would be the best way to get to the problem of this or should i just put a rag in the tank and strike a match to it.The local shops want 170 bucks to hook it up and another 100 to erase the codes,thats just a little to much for me right now any suggestions on what i could do to find out what started the miss fires,have already changed plugs, wires tps map and crank sensor still the same thing any thing im missing?|||Its either the coil packs or the crank sensor.|||Coil is bad.