Locking issue, always caused by the same query

[JamesN]

I'm getting the attached locking problem an awful lot and the load goes crazy in going over 100, and am wondering what the problem is and what I can do to help prevent it from happening?

It's always with queries such as this one:

                       SELECT MIN(postid) AS postid
                        FROM post
                        WHERE threadid = 21548
                                AND visible = 1
                                AND dateline > 1232080398