I have a 2004 9-5 with two, possibly related problems. First, sometimes the idle drops and the engine almost stalls out when I am sitting at traffic lights. It has never actually stalled, but sometimes the engine does bog down quite a bit for a moment or two. Second, at times when I am at highway speed, the engine surges and the turbo gauge jumps all over the place. It seems to happen only when I am trying to maintain my speed. If I accelerate or let off the gas, it doesn’t happen. I figured the first problem was just a vacuum leak, but the second problem made me think there is a problem with the O2 sensor or something. Does anyone have any ideas?