Happily, The problem is resolved.

For some time I was using a 2nd monitor but a few months ago I unplugged one and reset the display settings accordingly.

After following your advice to google around, I found several issues about maya & two monitors & getting the settings correct. I plugged the 2nd back in & reset the display settings and voila, my hypershade window works properly and consistantly now. So it was gfx card related.

Go figure!