Engine Belts And Pulleys problems of the 2007 Mercury Mariner

I have a 2007 Mercury Mariner I purchased in April 2007. The vehicle has been in the shop 4 times now. 3 of the 4 times because something has "gone out" on the vehicle I. E. The gas pedal, the wheel bearings, the tension pulley. I also found out from the dealership that that manufacturer never put the bolts on the top of my engine to cover the spark plugs. I continue to have a problem. These last three incidents have been within the last month and a half my vehicle has been in the shop getting new parts. I have always bought used. This is the first vehicle I have ever purchased a new vehicle and I find it hard to believe it should have this many problems. Everything so far is covered under warranty, but in 1,000 miles I have no warranty. I can't imagine how much it is going to cost if something else "goes out" on my brand new vehicle.