Question lock tables forever

've met this problem for the first time while trying to convert forum from phpbb2 to phpbb3 (rebuid searcher index), Today the problem came back. i saw message on page:
General Error SQL ERROR [ mysql4 ]

When i try to see in database what's wrong i see the same question in both case:

SELECT SQL_CALC_FOUND_ROWS p.post_id FROM (phpbb3_search_wordmatch m0, phpbb3_search_wordmatch m1, phpbb3_search_wordmatch m2, phpbb3_search_wordmatch m3, phpbb3_search_wordmatch m4, phpbb3_search_wordmatch m5, phpbb3_search_wordmatch m6, phpbb3_search_wordmatch m7, phpbb3_search_wordmatch m8, phpbb3_search_wordmatch m9, phpbb3_search_wordmatch m10, phpbb3_search_wordmatch m11, phpbb3_search_wordmatch m12, phpbb3_search_wordmatch m13, phpbb3_search_wordmatch m14, phpbb3_search_wordmatch m15, phpbb3_search_wordmatch m16, phpbb3_search_wordmatch m17, phpbb3_posts p) LEFT JOIN phpbb3_search_wordmatch m18 ON (m18.word_id IN (727, 1860) AND m18.post_id = m0.post_id) WHERE m0.post_id = p.post_id AND m0.word_id = 1058 AND m1.word_id = 1209 AND m1.post_id = m0.post_id AND m2.word_id = 1323 AND m2.post_id = m0.post_id AND m3.word_id = 1379 AND m3.post_id = m0.post_id AND m4.word_id = 1480 AND m4.post_id = m0.post_id AND m5.word_id = 3138 AND m5.post_id = m0.post_id AND m6.word_id = 3398 AND m6.post_id = m0.post_id AND m7.word_id = 4564 AND m7.post_id = m0.post_id AND m8.word_id = 6514 AND m8.post_id = m0.post_id AND m9.word_id = 13682 AND m9.post_id = m0.post_id AND m10.word_id = 16222 AND m10.post_id = m0.post_id AND m11.word_id = 21523 AND m11.post_id = m0.post_id AND m12.word_id = 52557 AND m12.post_id = m0.post_id AND m13.word_id = 62358 AND m13.post_id = m0.post_id AND m14.word_id = 211813 AND m14.post_id = m0.post_id AND m15.word_id = 211814 AND m15.post_id = m0.post_id AND m16.word_id IN (5638, 14395) AND m16.post_id = m0.post_id AND m17.word_id IN (870, 3739, 5821, 15213, 40664) AND m17.post_id = m0.post_id AND m18.word_id IS NULL AND p.post_approved = 1 AND p.forum_id NOT IN (41, 42, 43) GROUP BY p.post_id, p.post_time ORDER BY p.post_time DESC

Query never end, so after several minutes database reports: too many connections for user..., because this question lock tables, i can't even explain, because also hangs up. I try tomake myisamchk, to try reindexes table in databases, but it does not solve the problem. I haven't tried to remove words index and create it again, yet.

I have two forums, but problem exists only in one of them. I try to explain this question on another database (5.0.19) and problem exists.