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My 98 Subaru Forester has 77,000 milles. I bought it used and am unsure if the plugs/wires were changed at 60,000 miles. If the plugs/wires needed to be change wouldn't it run rough in both drive and in neutral? Recently the car has seen a 20% decrease in gas mileage and has begun to run rough when I stop at a light. If I shift into neutral the egine then runs smooth. The car also turns over a couple of time before it starts. Am I looking in the right direction or is there another way I need to go?  

Plugs and wires are a good place to start.  My Mazda lost ~20% mileage when running fouled plugs, plus (and due to) bad rings in a couple cylinders.  Idle was noticeably improved when I replaced the plugs.  Plugs/wires may not be your only problem but it helps to establish a known good baseline on the routine maintenance parts before proceding with additional diagnostics.  Hopefully some performance gurus will chime in and add additional insight.

Just my $.02