Why does the keyboard on my MacBook(s) stop working?

For the past few years across at least 4 MacBook Pro's or MacBook Air's, I have suddenly lost all input from the alphanumeric keys on my keyboard. The mousepad works, the computer runs, the arrow keys even work, but until I restart, the keyboard will not generate any input. The frequency of this occurring is a complex function of which computer I am using (the newest MacBook Air is worst), the OS X version I am running (Yosemite seems worst so far) and the task I am performing (marking up pdf files with Preview this weekend is causing a recurrence every 30-40 minutes). Lots of Internet searches have not uncovered any even remotely similar reported events.

Thanks in advance for any suggestions!

Rob