If you have a router, then its usual that there is a config somewhere that says 'disconnect on idle minutes' or something like that. Generally setting that to 0, or unsetting a flag, sorts this.
If its a USB modem, then somewhere in te softwatre that controls it there will be a similar setting.
If its being done by BT, god help you. Their support is completely usesless.
BT Broadband Routers are cack, and start to disconnect for no reason. May be a dodgy phone line, but unlikely.
I would go with either a setting, as other comments, or modem dying.
I swapped to a bought-in router, rather than the BT one for the same problem & it has workied fine since then. (my BT one was a small blueish black box, with abnout 3 green lights on the front)
The screen saver or the firewall could be switching it off as could the BT servers. Which got upgraded or messed with last?
Their patches tend to follow from the 2nd Tuesday in the month when M$ release their late latest.
I won't go near BT even changed the people who run the line because f their abysmal service when I first went online. If you look at ISP review's front page you might get the opinion that BT is the best but look who have been voted 3rd.
Search around and the 3 they recommend are not ones you might have heard of. No one else gets a worthy mention. Try them if you mind paying for help. And also consider changing your line to another company who might throw in a BB package to go with your rental.
Quite a few offer stuff for aboot =A325 -=A330 that includes line rental and some free telephony. HomeCall do something like that for about =A327. It sounds a lot but compare the Platinum service for AOL without the line rental and other offers. (I might have considered One-tel had they not been related to Bloody Gits.)
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