I recently bought my dream car, and now I want to get it in tip-top shape. It's a 1996 Fleetwood Brougham with 150k. I picked it up for 4 grand, which I thought was a steal. Well back to the topic, I changed the oil, the spark plugs, and ohmed out a few spark plug wires which indicated they were fine. But it has a cyclical vibration that I only feel in park. It's very slight, and lasts for a second, then smoothes out and comes back. Any ideas from an experienced owner?