Unfortunately, I don't know enough about networking or the build service to offer any more suggestions really. I did try manually throttling my connection and retrying a build and it did disconnect a few times before transferring me a built file.
It could be that your connection is good but unstable - hence the disconnects, but honestly I'm guessing. The only other thing to check would be if you can build one of the example files to see if it's project specific, or maybe use C3 in an incognito window in case any browser extensions (if you have any) have started causing an issue.
Ashley might have been able to offer more insight, but he's away this week I'm afraid.