I am on a 8mb/s line but my ADSL line and data is with Afrihost. Only paying Telkom for the line (R189). Since day one I got between 6mb/s and 7.15mb/s (on a good day) and if it is anything slower I reboot my router. At some stage a reboot did not solve my problem and I logged a call with Afrihost at about 9pm and the next morning at about 9am they send out a technician, from open serve, (Afrihost logs the call with Telkom and the Telkom/open serve technician attends to the call) and fixed something (at the pole where line is connected furthest away from my router) and speed was back to normal.
Maybe try and log the call directly with Telkom and hopefully they can send out someone to sort the problem.
http://beta.speedtest.net/result/6379137008