We've been having this same problem for months now as well. Our latest hypothesis, and what seems to have squashed the problem almost completely, is that it is Symantec Anti-Virus Corporate Edition's RTVScan. If we put an exclusion in for the %WINDIR%\SoftwareDistribution folder and then reinstall Windows Update, the problem appears to go away for good. Can't be absolutely for sure yet, as, like you said, it can be a bit random about when it happens. But it still might be something worth noting. I don't know if it is specifically Symantec's AV that causes this, it may be others as well. Hope this info helps you out.
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