I have a 94 LS 400, V8, with 180,000 miles. I have meticulously cared for this car over the years with oil changes every 3,000 miles and fluid and spark plug replacements every 30,000 miles. Very recently, the the engine started vibrating at around 1100 to 1200 rpm's. Above and below that range, it smooths out. What could be the cause of this?