Poor line speed

I upgraded my line from a 4mbps to a 8mbps line speed and I only get 7.8mbps when I'm on the phone with them after resetting data and changing setting and so on. <br> <br> This never lasts long because as soon as I'm off the phone with them it drops back down to 3.8 or 4mbps even sometimes down to 1mbps. I have phoned around 10 times explain this and I keep hearing different reason yet the problem never gets fixed. <br> <br> Before the upgrade my 4mbps line was running consistently at 3.8mbps. <br> <br> Will I ever get what I'm paying for?? <br> <br> My mother in law that's in the same area as me has never had a problem with her line since her upgrade, which leaves me wondering why I still have a problem with my line.