There are some steps you can do to pinpoint the origin of the problem...
First get the address of your router - the one that the ISP installed. Normally it's in your connection properties, as "Gateway".
Now open a command prompt (Start > Run > "cmd") and type (replace xxx.xxx.xxx.xxx with the address of your router)
ping -t xxx.xxx.xxx.xxx
(I'm not sure anymore if it's -t or maybe -i ... I think it's -t though)
What this does is ping the address without stopping (until you press Ctrl-C). Watch the output. As long as you receive replies, then there is a connection between you and the router. When you get timeouts or something then the connection is broken. Watch it for a minute or two. If you see breaks (timeouts or whatever) then the problem is in the connection between you and your router, otherwise if you experience that problem that you described but the ping stays stable, then it's beyond your router, and is the responsibility of the ISP.
Note that almost always, the problem turns out to be from them. So my advice is to save yourself the trouble and frustration and just assume it's their fault :) I don't know how many times I did diagnostics like that and the problem turned out to be from the ISP. It is better not to bother if you want to spare your mental health, I should not even have written this reply!
PS: I'm sorry I did not read the totality of your post. It seems you tried a lot of things and it does look like the problem is from their side. Pinging your router can still be useful to exclude problems from "your side" - although it's hard to be 100% sure.
What you could do is open two command prompt windows and ping at the same time your router, and a point beyond your router in their network - or a web address. If you see breaks in the "external" ping with the pattern that you described, but none in the ping to your router, then you can be sure it's from their network.
But anyway if the problem is not repeating itself anymore then it's gonna be hard to know what happened. It might be a problem with their antenna too, and it might be affected by the weather.
And besides, even if you pinpoint the problem, what's the use if you can't get past a level one support person who keeps telling you to restart your computer or such things... although it helps to be sure, you could then insist about contacting "level 2 support" or try your luck with writing a mail to noc [at] idm.net.lb