It sounds like you have a serious Aerial problem. In order to work properly, Freeview needs a good, correctly aligned, outdoor aerial. Generally speaking it won't work with indoor aerials at all and, unless you live near a transmitter nor will it work properly with loft aerials.
Alternatively if those fiver channels are the analogue ones you need to go into the menu and do a reinstall.
Assuming that you have lost the manual /support.sagem-communication.com/site/modele_fax.php?page=livret&prd=SAGEMITD62&pays=uk
BTW, it should not have searched every time you switched on.
T
March 2009