Consider the possibility of a third party software conflict. To help check this possibility, you can startup from a different account, e.g., Guest, or you can start up in Safe Mode. The strategy is to see if either of these options still lead to the same unwanted behavior.
Following your suggestion I created a clean user and did not see the problem. I then removed apps from the startup list on the original account but still didn't find a solution until I also started turning off apps that start from a preferences setting. The culprit was my antivirus program - it wasn't updating like it should and fixing that seems to have fixed everything else.
Thanks for the pointer.
contrary to my last message, I did not fix the problem but CT's advice did get me successfully down the right track. Despite having several decades in the computer industry, I'm a recent convert to Macs and I seem to have forgotten all the basic troubleshooting techniques I had under other operating systems.
In case there are other newbies who will find this interesting: I found the problem finally by using the console app to look for messages hitting at the same interval as the loss of focus. Putting up an application window next to the console, it was quickly apparent that it was a Microsoft sync process occuring every 9 to 10 seconds that was the cause. Disabling this through the Outlook 2011 for Mac preferences stopped the problem once and for all.
Thanks again to CT for helping me geting on track.