Lots of city miles combined with low octane fuel & no Techtron fuel additive (cleaner) creates excessive throttle body carbon build up (gunk) so throttle body must be cleaned.
To clean the throttle body remove the air intake hose and examine the intake and throttle body for excessive carbon build up if found then buy a can of throttle body cleaner labeled "safe on oxygen sensors, TPS's and catalytic converters" do not use carb cleaner make sure your engine is fully warmed up prior to using as when the throttle body is hot it dissolves the carbon easily and quickly.
The labels usually specify to park the vehicle at ground level (0 degrees, no inclination) and have the engine running but I disagree I instead park the vehicle on a hill facing down and shut the engine off and hold a rag below the lip of the throttle body and poor the stuff and use a tooth brush. You can also use "Sea Foam" the best engine cleaner:
Sidney(R) (tm) Repairs TV's,VCR's,home/car audio out of my apartment E-mail:sidneybek(at)yahoo.com Dartmouth,Nova Scotia Canada