If the car sits for 2 days and you go to start it it takes awhile to start (turns over just won't fire) then it will run like it is missing and clear up after a few minutes of driving. Also, after it fires there is a strong exhaust smell and grey smoke (could be unspent fuel but it does't smell like fuel) What could be causing this?

Background - replaced the plugs (NGK Iridium IV) and new wires (Vitek) about 7 months ago, along with the caps and rotors. Just repaced the O2 sensors a week ago thinking this might be the issue.

Any help would be great. Thanks!