When you say "Wich many websites" (Which I assume means with many websites) how many do you mean? Do you mean 25 and over, or 50 and over?
I don't understand why the number of URLs would affect this, I can't reproduce it with the basic 4 URLs.
There has been a couple of bug reports about the find URL feature however, one of which can be viewed
here along with a small script that might help you out for now.
Thanks for the report
Regards,