I recommend taking it to a car shop. Never take it to the dealership you bought it from, they will charge you a UNREAL amount of money to fix the problem. But it sounds to me like the engine is overheating, the reason why it seems like it runs on 5 cylinders is a safety feature that helps keep the engine cool until you can get the oil changed and have engine looked at for possible damage. I would recommend having your oil changed depending on how long its been since the last change. I also recommend buying some Sea Foam and pouring it into your oil (about 100 miles or so) before you get the oil changed. What the Sea foam will do is clean out all the gunk and buildup making the engine run cooler. And I also recommend pouring 2 bottles of Sea foam into your gas tank and running it every 4 fill ups. Do this and it should help your engine run cooler and get a little better gas mileage at the same time. Now it will smoke like a mosquito truck for about 10 minutes but that is normal when using Sea Foam.
