Just one point (because it may be she has no problem at all - call centres are known for just saying any old thing to make the caller go away), you say the speed test result was 6Mbps and the connection speed was 8Mbps.
6Mbps isn't too far off the maximum possible data download speed of an 8Mbps connection (the actual theoretical limit, taking overheads into account, is around 6.6 - 6.7Mbps).
You will never get a download speed of 8Mbps from an 8Mbps connection (it's just not possible - part of the 8Mbps is used for IP or ATM overheads).
What does the speedtester on this (the ThinkBroadband) site tell you?
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware