Hope someone can help: engine idles erratically & rough every few seconds, sometimes better, sometimes worse. So bad that when at a traffic light you feel it jerk every few seconds. If you accelerate on a bad idling "moment", huge lack of power, then suddenly all ok. Have fitted the correct NGK plugs & gapped them. Checked, there is no vac. leak, and I add injector cleaner often. (1998 900 turbo, auto) Any ideas?