focus mk1 1.8 zetec petrol occasionally revs high to 4000rpm when declutching; does not drop back to tick over and car will drive/ accelerate with no throttle applied! Eventually (10 miles or so) engine will return to normal. All quite disconcerting. When running 'normal' at cruising speed engine will occasionally feel as if it is misfiring for a split second. Restarting doesn't solve the problem. Any ideas?