I'll try and invoke the same error. What is your setup?
Down/upload timeouts are tricky as IIS has several protection mechanisms against denial of service attacks. Having said that, DMX uses another method of serving the file than the DNN core does. The latter gives you a 'hard reference' to the document by url so IIS takes care of streaming it to you. In DMX a procedure is used to pull the file from hard disk and stick it into the http stream. This means more coding, but allows the complete separation of the file from the downloaded entity (thereby permitting my own security model). I follow well documented and well known algorithms for this.