Look very carefully for vacuum leaks around the PCV valve.
The EGR valve could be staying open.
The MAF could be dirty and affecting mixture.
The IAC may be sticking.
A temperature sensor may be bad.
Could even be plugs and wires.
IMRC stuck open affects cold starts and power.
Basically you have to rule them out one at a time.
If the engine has not yet been serviced for intake cleaning, it's about time it was.
