Thank you for your reply. I welcome your assistance.
I think the problem will not lend itself to a quick solution. First of all, the problem exists with both FF and IE. Second, my inability to download exists on only a few sites that I have been able to identify. Unfortunately, the online malwarebytes scan is one of those sites where downloads fail. Thirdly (and I've not mentioned this before), at the same time that the original problem began I noticed a slowing down of loading of web pages, for those pages that do load. BleepingComputer loads. Yahoo.com does not. These are very consistent behaviors. I disabled my Norton AV and tried to load yahoo.com, still with no success.
I do believe this problem is related to the problem that Machiavelli has been working on, on the http://www.bleepingcomputer.com/forums/t/535289/unwanted-redirects/?view=getnewpost thread. He and I have already gone through a lot of diagnostics and attempted fixes there that might assist you, in case you've not already had a chance to review that thread.
Given the above information, especially the sporadic nature of the occurrences, and the fact that the problems occur with both IE and FF, do you still recommend uninstalling my AV? (I'll probably have to pay for a new license to reinstall it.) And do you still recommend the reversion and the reset? Should we try your other fixes first?