Slow speed - unexpected cause

For several days, my Sky connection speed dropped from its normal 7.6M to
about 4M.

While it was in this state, I was observing the stats and the snr was about
7dB. For reasons I won't bore you with, I'd been trying my dial-up
connection. I'd unplugged the dial-up modem lead from the filtered socket,
but left it sitting in the aperture. When I removed it altogether, the snr
immediately jumped to 14dB! On reboot, my speed was back to normal! I
assume the cable must still have been making slight contact, but still
wouldn't have expected such a drastic effect.

With the speed at its normal high, I plugged the dial-up fully in. The snr
dropped from 8.9dB to 5.8dB.

FWIW.

Allan