When i get the rss from some sites i get ??????? instead of normal text in Greek

you can see it http://www.patrabet.gr/patra/modules.php?name=Multiheadlines

Everything else is working fine.

The database is on utf-8

I tried to comment out

//if ($encoding=="UTF-8") {
//$string=utf8_decode($string);
// }

in function.php and this didnt worked....

When i change my encoding manualy to utf-8 i can see some sites working!

As I can see your site is "ISO-8859-7" encoded, at least you have between the head tags.

The feeds you are collecting are UTF-8.

What are you seeing right now is a UTF-8 encoded text seened as "ISO-8859-7".

If you change your browser encoding to UTF-8 [somewhere in menu->view-Encoding] you'll see that the text looks fine[for feeds] but might look wrong in other sides of you site.

The solution would be to focus on only one type of encoding[eventually choose to use only the feeds urls that gives you ISO-8859-7 encoded feeds] or make a function to convert the UTF-8 characters to ISO-8859-7 and apply it to the collected feeds[UTF-8 only].