Asterisk eat all cpu when meetme

i download asterisk and zaptel 1.2.6 and install them. when i create a conference room , the asterisk seems ok, but as time goes
asterisk will eat 99.% cpu . then all calls hangup. the time sometimes is nearly an hour sometimes serveral minutes . what 's wrong? is there anyone meet the same the problem?

i use ilbc as the codec and conference member is less than 8

Are you recording the conference conversation?

From the post above ,I find you change the meetme default codecs from gsm to ilibc.I am wonder how to chang the codecs from gsm to G729.Because my users all use G729 to call .So can use meetme which use gsm default.Thanks for any good hins.