This is, without a doubt, a fault of some kind.
The usual suspects would be;
A phone line fault
A hardware fault (either at the exchange, or with your router)
Something else has recently been plugged into a phone socket without an ADSL filter
An ADSL filter
As BatBoy and Robertos have suggested; try an alternative router (if you have one).
It may also be worth doing the usual tricks of trying the router plugged directly into the phone master socket (inside - into the 'test' socket), checking all extensions, swapping filters (using other filters if you have any spares).
Once you're sure it's nothing your end, I'd call your ISP and report the fault (mentioning your current connection speeds - up & down - and try to get through to them the line worked at full speed on ADSL1).
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware