Usally DoS & QoS stop speeds
once Dos was set as before and did not cost any trouble
and you have checked QoS to be off (inactive because older firmware was on by default) can not think right now any other trouble

also you can check at wan>>internet access possible settings modulation (to be adsl2 or adsl 2+)

what firmware are you on?
I've seen this with a few of our customers before and had to change the firmware to a "lower" modem code which resolved the issue.
I have also had an ISP say to be that Draytek Chipsets are not always compatible with BT Exchanges and to use them in Bridge mode....