00 Altima Misfire

I got a 00 Nissan Altima which seems to have a misfire. When you step on the gas pedal it wont respond. I already changed spark plugs, spark plugs cables, Fuel injectors. The last thing to change would be the intake manifold. But just to be sure Im asking.
Also it seems that the battery gets drained. So I changed the alternator and It keeps doing the same. Any clues of what it can be?:nono: