Strange, the WAG320N should be using a Broadcom chipset, which in my experience is the best. I have two Broadcom based modems, a TPlink TD8840 and Dlink 2500U (backup), and both are rock steady (±3.5Mbps sync) on my long marginal link (59dB attn) to the same exchange as you (Fourways).
So my guess would be that there are other environmental factors causing your problems (or you modem is partially damaged). Also to my knowledge Telkom run 3 different make/models of DSLAMs at Fourways, possibly you're on one that doesn't gel so well with the Broadcom?
To completely eliminate your current modem and chipset (DSLAM compatibility), I'd try to find something like an el-cheapo Billion 5200(S) or TPlink TD8840T, which use the TrendChip chipset. If that doesn't work then your line or DSLAM port/card are somewhere faulty, and its only Telkom that can rectify it.