The problem looks solved ! Having no problems now for two days ! :-)
May be related to installing about 20 Microsoft updates 2 days ago. Win32k.sys is now version 6.1.7601.18043 dated 2013-01-04.
The problem is back again since today (May 17th, 2013) :( :( after having no problems since end of January.
I would expect the cause to be new Microsoft updates, but I had nothing installed today or yesterday, last updates are from May 15th ....
A tool I installed earlier, 'Traystatus', shows me in the system tray the status of SHIFT and CAPSLOCK keys, and irregular either SHIFT or both SHIFT and CAPSLOCK are enabled as if they were pressed. Of course I press the SHIFT key regularly while typing,
but it should return to non-pressed state automatically. Sometimes it doesn't, and very occasional also CAPSLOCK gets activated.
%windir%\system32\wink32.sys is now v6.1.7601.18126 dated April 10, 2013.
Soooooo annoying, this ..