2004 Nissan Sentra

My car idles under 1000 rpm's and shuts off when at a complete stop, once gas is given it will ride smooth, Bought it 6 months ago, just replaced the spark plugs, thermostat, head gasket due to a torked spark plug broken off inside. Mechanic cannot find anything no check engine light and only code pulling up is multiple cylinder misfire. What can be causing this? It started shaking after I changed the coolant and replaced my radiator cap and I assumed it was the spark plugs since I failed to change them, they had corrosion when removing and now after getting it fixed Im here to idling before 1000. But before (which only lasted one day) It would idle once I turned the car on then it would stop after dropping to 1000 rpms's. What could be causing this?
March 11, 2013.

Have you checked for vacuum leaks? Have you checked the idle air control valve?

No vacuum leaks, the idle air control valve is my next step, but I am thinking of taking it into nissan

Mar 12, 2013.
Remove the IAC and have someone cycle the switch on and off to see if it is working.