I would agree with you that simply rebooting the Pro4060 is hiding the cause of the download problem.
Without having more detail about your installation and access to your systems it is very difficult to help you. When this system was install, I assume a Sonicwall CSSA configured it to your particular needs.
When you get the problem, if you stop downloading, leave the Sonicwall up, the TCP/IP connection to the download server should clear / timeout. Can you still surf the internet / send email, etc. Or does everything grind to a halt?
Are you Downloading from the internet / wan in to your system or are you uploading from your system to a server on the wan / internet?
If you are only getting this problem when a particular download occurs, I would suggest testing the download, does this download eminate from a standalone PC, multiple PCs or a server?, test one by one until the problem occurs, then isolate the problem PC / server. Where are you downloading from? could it be that there is a problem at the other end? Do you have a second internet path to the download server to test?. What protocals / ports are being used?
Has a specific rule been set-up for the download?
If you feel the problem is with the Pro4060 then I would suggest Sonicwall support, via mysonicwall.com. They will need a detailled diagram of your network, diagnostic report from the Pro4060 and fault conditions / protocal / ports sent to them, they will try to emulated in the lab what you have, just in case it could be a Sonicwall problem.
One thing to try, you should have a free trail of Sonicwall Comprehensive Security Suite for 30days, you just need to activate it. This includes Intrusion Detection / Spyware / Gateway AV. If you switch it all on, any problem with the download content / payload should be detected.
Do you have Sonicwall Viewpoint reporting system? it would help you greatly in tracking this problem down.
Sonicwall CSSA