I have a 93 cutlass that i just had in the shop and was told that the cat was bad and clogged so i replaced it and still the same issue, I was then told it was the map sensor, i replaced it, still the same problem, this car will one day drive fine and the next not start and if it does then it may just die and not start again for hours, ive checked the fuel pump, fuel pressure, changed fuel filter, i did all this casue i thought it was a fuel issue but theres fuel to all injectors and spark too all plugs , cranks fine but wont start, its kinda like the two wont work together. Help me , Im very good with cars and this has me stumpt im sure its a sensor but those things arent cheap and i dont want to just start replacing them ,, thanks to all . Tim

You didnt say what motor but....
I'd check the crankshaft position sensor.

Also check the coils and ignition module are working properly.

Thanks for the reply, its a 3.1 t v-6 , from what ive read from alot of other post i guess ill try that , if that doesnt do it anyother ideas ? Thanks