What could be causing my daughter's Ford Ka to over-rev when idling?

My daughter has a 2004 Ford Ka. It has developed an intermittent fault whereby it will often over-rev when idling or rev in "pulses", as if someone is fanning the throttle. It has now also started running off at lights/junctions, etc. I have taken it to two local garages who hooked it up to a computer but could find nothing wrong. They also checked for air leaks and drew a blank there too. They both suggested I took it to a Ford main dealer. Would this be a sensible option for such an old car and are they likely to have any more success diagnosing the fault than an independent garage?

Asked on 9 March 2013 by SR, Farnham

Answered by Honest John
First, pour a can of engine fuel system cleaner into the petrol tank and fill up with Shell V-Power petrol. If dirt or hydrocarbon varnishes are causing the problem, that may help. If that gives no joy, then it might be the mass airflow meter, the throttle position sensor, or the idle control valve - all garage jobs.
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