Drop in revs?

Hi guys, this might be a little vague but i'm just after a few thoughts on this. I was out earlier driving my car for at least an hour, it has been and was running fine. Anyway just as i pull into my driveway my revs drop really low to the point where my car cut out on me. Any idea's what might cause this? Normally my revs will sit about 750rpm and they dropped to 480rpm and cut out. I'm not that mechanically minded but if any of u guys could give me a few thoughts on what u think it might be it would be great.
 
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