Problems with exec()

Hi, Sorry if this has been mentioned earlier (I'm a new listmember), but I've run into a problem using exec() to execute a shell command (convert from the ImageMagick package, if that's of any importance). Basically, execution of the php script hangs the SECOND time I try to access the page. The first time, it runs fine. However, if I shut down the browser and open a new, fresh one, the script will work again ... once. Any idea why this occurs? And what can I do about it? thx for any help, //janne