How to test the internet

Sitting in an old office (building form 17th century, phones probably connected in the 80s at a guess, network cables laid in the 90s, something like that) office on the second floor and a server cupboard in the basements, with a ADSL modem next to the firewall.

Problem is, we are having the internet drop every so often, I would say maybe every half an hour or hour it drops for a second. Mostly it annoys the users because the webpage does not load, press reload a second or two later and the page turns up. One girl is using some archaic finance system which runs on access from another building, so looses connection and have to call the sys admin in the other building to be unlocked.

I have tried the standard stuff, the stuff I would use if trying to figure out if the line works at all. Things like IP tracing, logging ping requests and so forth. I have also checked the modem, its not noticing anything, or at least nothing that seems connected.

So, does anyone know any tools or utilities which I could use to just test loading webpages or test what is happening on the actual internet end. I think it is on the other side of the modem (because then its not my problem), but its hard to prove without a test. I want to run it on a laptop upstairs to prove that it is an issue, then downstairs connected to the modem to prove that it is an issue with the supplier (or tell my boss that a rat has chewed a cable and he needs a engineer in to put new cables in or something else exciting notmyproblemie).