Given that this was reproduced with direct file access to those PNGs, it doesn't seem like a "script timeout" issue as far as I can imagine.
Mouser reiterates the critical issue: it's not necessarily surprising that the transfers should break or whatever, it's the fact that when they do, the browser doesn't realize it and subsequent attempts to download the same file (from the same location) don't trigger a re-download but instead just use a cache.
This makes me wonder whether it may be some kind of in-between ISP caching. I know someone else mentioned that. Very hard to say indeed.
It's also important to note we may be looking at 2 separate but related issues. On one side certainly there are occasional/intermittent transfer issues, as seen with the PNG issue pointed out earlier in this thread. These would also be likely at the root of, or at least involved in, the reason the installer download problems are occurring. So that issue certainly seems to need investigation and resolution if possible, and it seems that can only be either server-side or at some link in-between since it was reproduced simultaneously on multiple distant systems with (presumably) different Internet providers. It would be interesting to test packet loss to the server from a machine that is currently experiencing the slow download issue. It's also important to note it *seems* to be isolated to particular files on the server when it occurs, which is an additional twist which really makes it confusing, but at least suggests that it is more specific to the server than an in-between link (maybe...).
And then on the other side is the issue of the browser not realizing - or not being properly told - that a download is actually not complete when the user requests to download it again. This seems to occur cross-browsers so it does not suggest an issue in a particular browser's caching mechanism. While there may be such an issue common to many browsers, it seems somewhat dubious given they all work somewhat differently (for example Firefox uses temp files in the same directory as the download, while IE uses the system's temp folder and then copies the finished file to the destination folder when complete). Another possibility is some kind of caching in-between, likely at the ISP level (I don't know if the higher-level interlink providers like Level 3 do caching though). This would explain why it happens cross-browser. However my own experience with this issue suggests in fact that is not always so consistent. Sometimes using another browser *will* help. So it's all very confusing.
Bottom line I think the intermittent transfer speed issues from the server are a clear problem and need to be resolved. If they can be fixed it will at least minimize the chance of the "browser download confusion" that is the 2nd part of this issue.