Mysql not using correct indexes and may be a bug

[cobaku]

PHP Code:

SELECT threadid, title, prefixid, lastpost, replycount                         FROM thread AS thread                  WHERE forumid = 1393                                 AND visible = 1                                 AND open <> 10                          ORDER BY dateline DESC                         LIMIT 6960,87; 

when i execute this query mysql uses a random index named forumid
and
the query executes in 10.17 sec

when i force an index like below

PHP Code:

SELECT threadid, title, prefixid, lastpost, replycount                         FROM thread AS thread     FORCE INDEX(zarchivethread)                    WHERE forumid = 1393                                 AND visible = 1                                 AND open <> 10                          ORDER BY dateline DESC                         LIMIT 6960,87; 

when i do above and force index
the query executes in 0.17 sec


My question is is there any way in mysql to chose correct index (automaticly) i dont want to change php 1 by 1

this is really important for me, i would be happy if i get an answer.
thanks alot.