It happens 24/7 as far as I can see - I was downloading something which usually would take a few hours overnight the other night, and it wasn't even halfway done by the time I got up. It also happened all day during the weekend.
I'll try switching everything off when I get home. Oddly though, whenever I've restarted my router it seems to restart the pattern - I always get the 7-odd minutes of good connectivity followed by the 25 minute slowdown. If it was external you'd presumably expect that I'd turn my router back on during the middle of at least one of the slowdowns.
I'm in a quiet little village nice and near to my exchange - nothing heavy duty ever goes on here.

Edited by deleted (Wed 23-Mar-11 10:29:33)

In reply to a post by Masterdog:

nothing heavy duty ever goes on here.

Doesn't look like electric fence to me - pattern's wrong.

Can't be a level crossing or electric trains - too regular!

Noisy fridge?

Please reply re: alarms, oil tanks, and testing the extension sockets.

Have you tried a different router?

Okay, a bit more information. Turns out the problem only occurs when my main PC (connected via Ethernet) is on. When it's off, the internet performs normally. As soon as I turned on the main PC (and waited the requisite 7ish minutes), the problem came back.
I'm going to boot to an Ubuntu live CD to try and determine whether the problem is with hard, firm, or software. I did mess around inside the computer the other day so I'll probably poke around in there too.
Thanks for the help all so far...not had a problem like this before!

Hmm. Noisy switchmode PSU probably.

Could be either the PC or the monitor - does the monitor have a seperate power brick? Move the stuff as far away as you can from the router and it's cabling, and maybe try a surge filter on the mains supply to the router.

At least you know what the cause is.

Well it worked okay on the other devices when the problem computer was on but not connected to the router. It also seems to be running along very nicely at the moment using Ubuntu. Guessing it's a software or driver issue then?