[16th hippy]

for starters, engine size, make, model and so on is really needed. just guessing here, but assume you have a malibu as you are posting here, and you have either a 3.1, 3.4, or 3.8(if they put those in malibu's) since you have done the intake gaskets....in any case, this info is always helpful.

hissing sound is a vacuum leak most likely. u didn't mention if it was coming from front or rear, but if front, would suspect fuel pressure regulator vaccum line, or somewhere close to that area. either way, check ALL vacuum lines for leaks and repair as needed. regardless of where this is, it needs to be repaired first, then if misfire and lack of power are still present, then go from there. double check spark plug wire routing, and also, double check coil posts. if any are corroded, replace that coil pack. also, check all wiring for EGR and MAP sensor. could be you forgot to hook the EGR pipe back up and is why the lean codes, misfire code, and EGR code all at once, and this could have triggered the MAP code. this could be causing the o2 heater to work overtime and setting a code, or it could just be that it was time for the heater to die. post back what you find and/or what fixes it!