rpm at idle drop

Hi guy's got a question.
in the last few weeks i have notcied that after runing my car for about 30 minutes to a hour the idle will float around a bit . usually stays at 800 or so but the last few weeks it has started to drop to 700 even down to 500 and feels like it wants to stall then goes back up to about 800 or 700 range. car is totally stock 04 mystic 700 miles. thanks in advance for any ideas.Plan on taking to dealer next wednesday to have them take a peek going on a long run next saturday and don't want to take a chance fo breaking down someplace.