I bought my D-Link DSL-300G+ last November. It worked great up until near the beginning of this year.
What happens is the modem connects to BTOpenworld successfully. I can browse the web for a while (anything from 1 minute up until a variable amount of time) and then all Internet access stops working.
The modem still thinks it has a PPPoA connection to BTOpenworld. I can still get to the web admin page for the modem. I can still ping the modem's IP 192.168.0.1. But I cannot ping an external web site address. So it appears that there is a fault with the modem. I've already sent the modem for repair, but D-Link just re-uploaded the same firmware version and returned it to me.
I know my ADSL line is OK because my USB modem works - and I've also tried a mate's Netgear modem and it gives a 100% reliable Internet connection.