Since approximately 11:30, Internet access has been unreliable, with high latencies and packet loss observed. It would seem that the problem is caused by multiple link failures within the TENET network. The problem has been reported to TENET and we are awaiting feedback from them.
Update, 1:11 PM: TENET report that three links in their network within the Eastern Cape have failed. They've reported the failures to Telkom.
Update, 2:39 PM: at least one of the links was restored at 14:15, allowing traffic to return largely to normal.