When my friend had this problem it transpired that her line wasn't capable of keeping up with a MB connection. she had to down grade to a MB connection and no problems with losing connection since.
Has wanadoo arrange tracerts to be carried out. I had problems with my onetel connection, when I up-graded it didn't kick in, so they sent me a e-mail with instructions to carry out 3 tracerts and a speed test, to be sent back to solve the problem. I'm not sure what the problem was or what how the tracerts would solve the problem, but it did but I've received no explanations whats so ever..
But you can check your BB capabilities by going here click here, this will give you weather BB is available and if you click onto the adls box to the left after you line has been checked, then this will give you the information for your exchange, and weather you can get MB via that exchange.