I noticed this first on a Business Hub. If you download something big (CD or DVD image for example) and the download is using the full bandwidth available to you, DNS lookups stop working on the hub (whether its through a browser or a tool such as nslookup). You can still ping WAN IPs and do an nslookup if you specify another server, and internet sessions that don't require DNS (where you know the IP) aren't affected - they keep working fine, if a little slowly ~(which is to be expected).
It started happening on a several-year-old Business Hub about a year ago, and I thought it might just be the hub. In the last month or so though I've started using another Business hub of the same model (BT2700HGV), and it has the same issue. I've also moved to BT at home recently, and now have a Home Hub 3, which also does this.
Has anyone else experienced this with BT or other hardware? I've never experienced it with other modem/routers, including really old ones that can barely stay up for 24 hours without crashing.