Have you still got your ring/bell wire attached (terminal 3)?

When BT launched the iPlate they used noisy fluorescent lights as part of the testing!
So you could remove the bell wire or fit an iPlate - assuming you haven't done either of these already.

Also some routers cope better than others. 2Wire are the only company I've come across that actually mention this in their guides.

2Wire:
"It also minimizes common interference found when other
devices (such as dimmer switches or fluorescent lighting) are in contact with the DSL line."

Finally changing the light choke or the actual light could change things if it's failing!

No bell wire attached anywhere.