My 1990 Century Iron Duke sits at our beach villa year round. This weekend I cranked her and did some highway driving. Noticed at first she was stumbling at idle, figured a good Italian tune-up would do the trick, no dice. After the highway run still almost stalling at idle. Past summer I changed plug wires (not plugs), yesterday I shot some carb cleaner down her throat, no change. Drove her 2 weeks ago and this was not an issue. Any ideas? TIA