How many errored seconds is acceptable?

I've recently been noticing a lot of web pages stalling whilst loading and
checking my router I discovered that BT have upped my line speed from 1Mb
to 2Mb without my realising it. It's showing a local SNR of around 9dB and
I'm consistently getting around 2 errored seconds per minute downstream.
Could this be the problem? Can/should I get my ISP to get BT to put me back
to 1Mb? My account limits me to 500Kb/sec at the ISP end anyway so 1Mb/2Mb
line speed makes no difference to me.

Unfortunately I don't have any comparison error figures for my 1Mb
connection, though I know the SNR was about the same which suggests the
line quality may have been improved when I was upgraded.