Not sure about your current issue but if the car is over 5 years old (2007 says it is) then the coolant should be drained, flushed a couple times with distilled water, and refilled with new coolant. It's only good for 5 years (or 100,000) miles. I changed it on my 2008 back in the early spring at around 68,000.
I'm not sure about the 2007 but I don't think the 3.5L has a PCV valve that is replaceable.
You could do the spark plugs (make sure proper gapping). I haven't done it on mine yet at 71,000 this is a cheap fix though not quite as easy as it was on the wife's 4 cylinder 2008 Vue. Maybe it would help your issue? Plus it depends how long you expect to own this car.
Any check engine light on? Scan it to see if anything comes up. You may have a leak in the fuel system somewhere.
2008 G6 Dark Gray Metallic SE V6