it was EXACTLY the same issue as you even down to there being no packet loss to the Cable modem.
apologies for the small reply but on holiday from ntl for a couple of weeks and away from home having to make do with 9.6k internet via mobile
there was a raised fault reported on your ubr as a result and to be honest i left before a resolution was found... so im not sure if was has even been found... but it sounds like it has
