IPStream and Datastream are not the same thing, and it is highly likely you are now on IPStream Connect which is a different thing again.
There are a few possible sources of exchange contention, then BT Wholesale Cloud can have occasional hiccups, then there is the handover at the IPSC nodes that can be under-configured, possibly by either of BT Wholesale or the ISP.
Next up is the ISP's own network and peering arrangements.
It would help if you tell us the ISP and your exchange. Plus is it just the throughput that drops, or does latency go through the roof as well?
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
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