Wonder if anyone can offer advice; my 85k mileage Ford Mustang 2000 GT has recently started to run at very low revs when idling. It will run app 800 revs whilst idling (at an intersection/stop), and then every 20 secs or so, increase noticeably to app 1.1k revs for a few seconds, and then drop down to app 400 revs when it sounds (and feels) as it is about to stall. Lasts for a couple of seconds or so, then back to standard 700/800 rpm.

A little perplexed? Have not moved to a new gas, oil and filter change 3k miles ago, and no real problems ever in its life to date. Keen to learn more as I now have the car with me here in the UK and there are not a lot of Mustangs around :(