Hi Garo,
This is the symptom of a faulty Fan Oven Element. You can see a list of Smeg elements which are available from us here - copy and paste in to your browser:
http://www.ukapplianceparts.com/elements.asp?search_fd0=Fan%20Oven%20Element&search_fd3=smeg&multisearch_fd3=5
You will need to identify what model of oven you have and then search for your model from the list.
Replacement is fairly straightforward. If you need some assistance contact us by email via the web site.
Lee
December 2006