Converted query trouble

[gordy]

Can any database guru's understand why this query is failing (I eventually have to kill the process)

SELECT  xf_attachment.attachment_id, xf_thread.thread_id, xf_thread.title
FROM xf_thread
LEFT JOIN xf_attachment as xf_attachment
ON(xf_attachment.data_id = xf_thread.first_post_id)
WHERE xf_thread.thread_id
AND node_id = 5
GROUP BY xf_thread.thread_id
ORDER BY thread_id
DESC LIMIT 15;

It is based of my vB query which runs as a snap, less than a second

SELECT  attachment.attachmentid, thread.threadid, thread.title
FROM thread
LEFT JOIN attachment as attachment
ON(attachment.postid = thread.firstpostid)
WHERE thread.threadid
AND forumid = 22
GROUP BY thread.threadid
ORDER BY threadid
DESC LIMIT 15;

I have this cron'd to dump out to a static file every hour