I had a similar experience after moving up to max. I came to the conclusion that plusnet could do very little as there seems to be so many variables that can cause problems.

My solution in the end was to replace my old belkin router, which had worked well on 2meg service with Vigor 2800 (wanted VOIP and print server). From the day I plugged it in, I've got a reasonable stable 3 meg service. So the line was fine, plusnet were doing what they said they would do....it was the hardware couldn't cope with MaxDSL. Who's fault is that? I don't think its plusnet's

I feel sorry for plusnet as well as the other ISPs as they take a lot of flack for a difficult BT product that they have little control over.