Image Headers break when image is out of bounds

I've got a bit of a weird one here. I'm using PHP to output an image. This works fine when I output as a file, but when I try to feed it directly to the browser it fills the screen with random characters. I know I've got the headers right, as it works some of the time. The only thing I can spot when it does fail is that some of the image drawing commands are essentially drawing off the edge of the image area. I've checked the error logs, and the only errors/notices I can find today are a bug in phpMyAdmin and a missing favicon file, nothing to do with this script. Has anybody ever noticed a similar problem? Thanks, Ash http://www.ashleysheridan.co.uk