You probably need to update your web.config file to increase executionTimeout (the time an asp.net page can run for) and the maxRequestLength value (the maximum size of a file in kb, the default is 4096=4MB) i.e. the following says to give a script 10 minutes to try to upload files up to 10MB (10240)

 <httpRuntime  executionTimeout="600"
                               useFullyQualifiedRedirectUrl="true"
                               maxRequestLength="10240"
                               requestLengthDiskThreshold="10240" />

I was able to solve part of the issue with a little more digging. Here's what happened...

While looking at the web logs, I noticed that the following error was showing up a lot.

A worker process with process id of '3856' serving application pool 'sitename.com' has requested a recycle because it reached its virtual memory limit.

So I tried to download the file again, and noticed that it was the file download that was causing the memory maximum to be reached. I had the site set to auto-recycle whenever the virtual memory reaches 200 MB, and somehow it's breaking that limit. So I told it to only auto-recycle when the physical memory max is reached (150 MB) and the downloads started working immediately! However, is it normal for a DNN portal to consume 200+ megs of virtual memory?? At any given time, there's only 2 or 3 people browsing the site. There's nothing fancy on the site, just pictures, text, and PDF downloads.

Robert