I noticed that when the outside temp is warm (i.e. 70+ deg), my 4.8L Vortec runs smooth. But when it gets colder out ( I live in northern Ontario ) I get alot of vibration out of my motor. I checked my iat sensor and it is fine. Could this be an air leak somwhere on the intake?I am not hearing any sort of vaccum leak anywhere. Any input would be appreciated.

V8's are very smooth engines. If its vibrating, its probably misfiring.

Haven't heard of any real issues with the new Vortec engines, so don't have any good advice, other than when it gets cold out, try to figure out which cylinder is misfiring and go from there.....