WSUS connection failure

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On a SBS2k3 R2 site with ISA started getting repeated failed downloads from WSUS - the ISA log indicates that "A connection has abortively closed after one of the peers sent a RST segment"
Can anyone explain what this is & more importantly how to cure/stop it, as it appears to happen for several attempted downloads & repeats itself multiple times per hour! - the only additional information is that it shows varying amounts of data sent & received both in the region of 300,000-500,000bytes but I assume as it failed the update has also.

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