Are you running the recommended fuel grade? I believe it is 91 that is required but your manual will confirm this.

 

During start up when an engine is cold the PCM confirms engine and outside temperatures to ensure proper fuel/air ratio and adjusts both (basic choke) during warm up and releases vauume, timing, ratios and other semi procedures. It sounds like one of 2 problems.

1. Fuel octane is too low or crapy gas. Move to 91 octane or try a different station.

2. There is also the Air Charge Sensor that tells the temperature of incoming air mounted on the intake manifold front passenger side midway down.

A leaky "belly pan gasket issue" could cause this I guess because of air/fuel mixture so look into the intake like N8 is talking about but understand the some oily film is normal because of the PVC valve. Which also brings up another option. ( PVC Valve )

Also when was the last time you checked your TPS sensor?????
Tuneup as well?????