The root cause of the problem is with the advanced inventory scanner module (amAdvInvNT.exe) from the AM agent. In order to inventory a list of operating system updates on Windows, the module parses output from a WMI query. Due to a preset timeout in the code, sometimes the WMI query would return within the length of time the inventory module would expect, and other times it would not.
As the AM agent works using a delta inventory (difference inventory) from the previous inventory, the AM agent would in one instance create a huge delta, to re-report all the operating system updates, then in the next instance erase all the operating system updates. Hence the AM agent inventory is observed to be fluctuating.
As a result of this, the Engine on the Domain Manager that is collecting these inventory files, is busy writing, erasing and rewriting this inventory, each time the AM agent changes it, overall contributing to poor collect task performance. It doesn't help that the System Update inventory is typically the single largest section of inventory reported by the AM agent.
Note: The AM agent reporting of operating system updates should not be confused with the software inventory scanner module (amsoftscan.exe & amswsigscan.exe) of installed software, including Windows patches, nor Software Delivery installation records reported by the SD agent. These records are not affected by this problem.