Huge looped search query breaking down the site at vb3.7.3

[socialvisionsbcn]

I have found at mysql slow-query-log a query that includes EVERY theadId in the site

20392 dbuser host:60101 dbname Query 355 Sending data SELECT thread.threadid FROM thread AS thread INNER JOIN post AS post ON(thread.threadid = post.threadid ) WHERE post.postid IN(1109,1267,1294,14,16,43,44,49,393,882,1272,3089 ,3121,...-all existing threads at forum-)

In my logged forum the query goes up to 2Mb long and of course compromises server and sql performance.
New Relic was not able to trace it (due to its length, I supose) so the issue took long to be identified.

Seems like it is generated at original search.php file, with some special conditions, but with original vb code.

I would gratify solution/explanation for this. Any experience with this long looped threadId queries?

Thanks in advance.

My configuration currently:
PHP Version 5.2.17
MySQL version 5.0.92