Yup. Having the same issues in Centurion.
My Ping before 4PM is 14ms, after 4PM is between 300-400ms.
Speed drops from 3.4Mbps to 0.20Mbps - 2.00Mbps after 4Pm for the entire evening
I think Telkom DSMS service is to blame, see link below.
Ok ,so a Technician phoned me, problem is congestion on the exchange.
They will be upgrading it, but only in June, and after that it will support 10meg as well.