thanks tweak'e, have replaced fuel lines and clamps thinking it could be an air leak issue, no visible fuel leaks, haven't lost power, revs smoothly, fuel economy still the same. Replaced the MAP sensor just in case. The old beast has quite a few k's on it, 540k (had it since new). I've been googling but can't find anyone with a similar problem. All I can think of is a dirty injector or 2. If I rev slightly at idle, say 50rpm, it goes away. Any thoughts would be greatly appreciated.