Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /home/xxx/public_html/include/ust.php on line 406

it was start when there are bigger then 400.000 record in the tag table. im refreshing my website sometime happen sometime not happen :S

Need help this keeps running an error. Anyone know why?

$etiket=mysql_query("select * from tag order by rand() limit 50");
while ($eti=mysql_fetch_array($etiket)) { $et .= '<a href="'.$eti['tr'].'-video'.$eti['id'].'.html" title="'.$eti['baslik'].'"><font color="#'.randchar(8,"hexcolor").'">'.substr(trim(strip_tags($eti['baslik'])),0,22).'</font></a> ';

Please Help..