I have a 2005 Santa Fe 3.5 AWD that has an intermittent running problem. Originally, the engine would miss at times (not all the time) and the check engine light would come on. So I did a scan and PO300 random misfire, PO306 cylinder 6 misfire and PO304 cylinder 4 misfire came up. I installed all new plugs and wires and that seemed to correct the cel from coming on. However, at times the engine still seems to misfire (but no cel light) especially when I am in drive with my foot on the brake and even more when my air conditioner is on. In fact, the other morning, a dark 4 am, I was pulling into a parking stall at work and when I braked, the engine did its little stutter dance and I noticed that my head lights went from bright to dim several times and the tac would flucuate up and down by about 5 rpm's. All of this stopped when I placed the car in park. A subsequent alternator and battery test proved they were ok. Any thoughts out there? I am perplexed!

 

This may not apply, but should not be overlooked. Since something occurred when stepping on the brake pedal, it would be a good idea to confirm there is no problem with the stoplight circuit. A direct short should cause a fuse to be blown, but there may be a ground connection problem and that could affect other systems on the vehicle. Something is pulling the system down and one of the things it is affecting is the headlights and stop lites. They are on separate circuits, but probably share the same ground connection. Check all the grounds coming from the battery negative cable to the sheet metal and engine.