'not in' query is very slow how to change it?

[kevin smith]

query:
mysql> explain select `question` from questions where `iid` = '2' and `id` >= '14' and `question` not in (select q from questions,occur where q = question and occurid = 'i25085126' and occur.a <> '' group by q order by occur.id desc) and `active` = 1 order by ord asc limit 0, 1;
+----+--------------------+-----------+-------+---------------+--------------+---------+-------------------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-----------+-------+---------------+--------------+---------+-------------------+--------+----------------------------------------------+
| 1 | PRIMARY | questions | range | PRIMARY | PRIMARY | 4 | NULL | 48 | Using where; Using filesort |
| 2 | DEPENDENT SUBQUERY | occur | ALL | NULL | NULL | NULL | NULL | 129015 | Using where; Using temporary; Using filesort |
| 2 | DEPENDENT SUBQUERY | questions | ref | idx_question | idx_question | 153 | RiskStomp.occur.q | 5 | Using where; Using index |
+----+--------------------+-----------+-------+---------------+--------------+---------+-------------------+--------+----------------------------------------------+
3 rows in set (0.00 sec)

Execution time: 13 sec

CREATE TABLE `questions` (
`id` int(11) NOT NULL auto_increment,
`iid` int(11) default NULL,
`question` varchar(150) default NULL,
`dt` timestamp NOT NULL default CURRENT_TIMESTAMP,
`active` varchar(2) default '1',
`ord` varchar(3) default NULL,
`rule` text,
`descp` text,
PRIMARY KEY (`id`),
KEY `idx_question` (`question`)
) ENGINE=MyISAM

CREATE TABLE `occur` (
`id` int(10) NOT NULL auto_increment,
`dt` timestamp NOT NULL default CURRENT_TIMESTAMP,
`occurid` varchar(50) NOT NULL,
`qid` varchar(10) NOT NULL,
`aid` varchar(10) NOT NULL,
`q` text,
`a` text,
`user` varchar(10) default NULL,
`ip` varchar(20) default NULL,
`raw` varchar(2) default '0',
PRIMARY KEY (`id`),
KEY `idx_occur` (`qid`)
) ENGINE=MyISAM

Thanks