Intermittent difficulty starting up and idling properly

Problem does not always occur, but when it does the vehicle will either start up sluggishly or require the accelerator pedal to be depressed for the engine to turn over. Once running, the car does not idle properly with the tachometer ranging between 1,500 to 2,000 RPM's. During this the car will want to shut off unless the accelerator pedal is depressed and held until a sudden spike of roughly an additional 1,000 RPM's occurs. After this spike the RPM's will remain at the number it spiked to and the vehicle will then idle on its own.

The check engine light has never triggered over this problem and, as I said before, it only occurs sometimes. Also, when this happens there doesn't seem to be a pattern or relation to the time of day, ambient air temperature, or even the length of time the vehicle has been shut off/parked.

Based upon what details I provided, any advice or suggestions as to the likely cause of the problem?