I'd start by looking for leaks. Try spraying brake cleaner on your intake boot while the engine is idling and listen if the engine revs fluctuate. Chances are it's your CCV, but do the leak check first. If you can't find any leaks(cracked or split intake boot), replace the CCV(crank case vent valve) and clean inside the fitting on the back of the valve cover where the hose attaches. You might as well remove the valve cover and replace the gasket while you're at it. Also, check your spark plugs.