ISDSL "broadband" IS Darn SLow

It's high time that Internet Solutions put the "S" back into ISP. I can understand Telkom being completely useless, since they are a bumbling state monopoly, but Internet Solutions is part of a commercial organisation listed on the London and Johannesburg Stock Exchanges. So what's their excuse, apart from greed?
Take a look at the traffic graph above. I have a 384kbps ADSL line. In SA that's regarded as "broadband". It's Monday and I'm running 3 simultaneous downloads, two from Audible.com and a podcast from twit.tv. It's 4am in the morning in the USA, so these web sites are hardly likely to be overloaded. Yesterday I downloaded a similar audiobook from Audible, and the download went as fast as my line would allow, i.e the yellow line maxed out just above the green graph line.
Today, if I stop the podcast download, the graph pattern gets even slower, and the line runs at about 50% capacity.
I have queried worse performance than this once before, and ISDSL have been unable to explain why this is happening. They also can't or won't provide me with a proxy server that I can use. When I used the Telkom/SAIX ADSL offering, I could connect to dsl-cache.saix.net:8080 and podcast downloads would often go much faster, because the podcast was already cached locally. ISDSL have a bunch of "invisible" proxies, but they can't tell me which one I am connected to. This has caused other problems, for which they have been unable to provide a "solution".
Everyone said that when the Seacom cable was connected in SA that we would have more bandwidth and lower bandwidth costs. Clearly this is not happening, or the Seacom cable is already oversubscribed. My guess is that Internet Solutions wants to pay for as little international pipe as possible, and just screw their customers like Telkom does. In the past they blamed Telkom. Who are they blaming now? Their shareholders?

Update: I called the Telkom ADSL fault logging service, and they "recreated the port" and then during the next few minutes we logged 5 ping timeouts between the modem and its own gateway, 196.210.152.129. I stopped the pinging and ran the speedtest.net and pingtest.net tests, results shown here. 384kbps works out at 0.375Mb/s, and I'm measuring 0.32Mb/s locally. The technician reckons if I upgrade to a 512kbps the problem will get worse.
Update 24 Jan: Downloading books from Audible.com definitely works better on a Sunday
Update 26 Jan: Windows update at 10pm on a Tuesday evening is back to its usual nonsense.
Of course it's Microsoft's fault, and not the local caching server. That's why there is no "S" in ISP.
Update Wednesday 27: Still slow downloading 2 books simultaneously from Audible at 10pm:
I did notice that when I used Firefox to go to a gdgt.com page with plenty of pictures on it, then the download speed maxed out while the pictures were loading. So are they just sluggish with single or double download threads, but fine with, say, 4 or 8 download threads? The mind boggles.