I think I'm gonna be a barer of bad news here because I'm pretty sure your line can't go faster then a 2meg speed.
If you are not syncing at 2 megs no way you will ever be able to sync a 6, unless Telkom upgrade the internet in your area which may be p
You may even need to downgrade your line to a 1 meg line for it to be stable but get a technician out to tell you whats going on and make sure they reset the port on your line.
Go to this website
http://www2.telkom.co.za/checkadsl/check.htm and put in your phone number to check what speed you can get.
Not syncing means your line is trying to stabilize but cant so it jumps up and down. When this happens you lose connection and then regain connection at a lower speed so when you say 1.72mbps that's not a good thing.
If all else fails try amobia satellite internet