My browsers simply don't download HTML now and again

Sounds like a networking problem, where you occasionally lose the connection.

Have you tried resetting the router? Are you using any proxies? Are your DNS servers the ones your ISP gives you to use?

Also check signal strength. If you stray too far from the router, the signal will become weak, and you will get alot of dropped/corrupted packets. This can lead to all sorts of weird behavior when trying to use the connection.

The computer is connected directly to the router, not wireless. And i checked if it's a DNS problem, since during the problematic times my getting into my router management locally via IP (no DNS resolving necessary) suffers from similar inabilities to connect correctly, then the problem isn't DNS related. When I connect to the router via the browser during the spurts, then like with other websites it may not show the correct HTML format for the page, just the text without frames, colors or what not.

You are describing pages loading without CSS files, which is a common occurrence with networking errors and packet loss but that can also be caused by software issues (plugins , settings etc). Thing is, you say it happens on different browsers, so it seems to point to a networking issue.

If you have this issue even when connecting to the router's firmware then it sounds like the issue is below your router, but without any more information it's hard to say why.

Have you tried pinging when you see this error, to see how many packets are getting through? And do you have any p2p software running when you see this error? I've seen bad p2p software setting cause packet loss through sheer volume of packets they use under some settings.