Classic asp timeout at IIS8

the asp works at IIS6 very well

there are 4000 emails sent by script with writting log to a file at the same time.

after moved to IIS8, there is timeout issue and it runs very slow

some posts said the slowness is because of asp file IO operations which i accepted.

when send action is clicked, it has timeout after 23xx emails sent,

then 2nd click, timeout at 8xx emails sent

then 3rd click, timeout at 4xx emails sent....

it did not be solved after reset of web site/app pool.

any help pls.   thx.