ADSL line intermittent

After increasing my line speed from 4mbs to 10mbs, my ADSL has been very intermittent. Getting anything from a few Kb, to 6mbs. Logged quite a few calls to the call cente, and I keep on getting the same story, switch off your router, wait, switch on your router, and there your problem is fixed. I am afraid not, eventually called my ISP, and they picked up a problem at the Telkom exchange. Called the call centre again, to report the problem, now passing the buck, telling me it is my ISP where the problem lies. This is after I just got off the phone with my ISP for about 20min to find the problem. Eventually got a refrence number, and said they will send a tech guy. Well it has been 2 weeks, and not a word, and my problem persists. I am paying for a service, why is it not being delivered.