After a week or so of troubleshooting, I've finally found out why iTunes Match would continue to run, hang, then re-run, non-stop.
Seems it is the result of doing a full 'Erase & install' of Mountain Lion OSX 10.8, over the top of my existing Lion OSX10.7 install.
Had I simply upgraded, I wouldn't have had this issue. Clearly, logging into iCloud is not enough to tell your Mac that this is now the default iTunes library. The truth clearly isn't in the cloud.
To kick iTunes Match back into line, I had to Turn Off iTunes Match and Deauthorise this Computer under the Store menu, then not select Turn On iTunes Match from the menu, but select iTunes Match from within the store home page.
Frustrating & non-intuitive . I could not find one single Apple support document to troubleshoot this problem.
We still have a way to go before this all just works.