Irregular behavior of CTRL (left) key in kbd shortcuts

With my old keyboard I had an annoying problem:
sometimes (more often than not) an keyboard shortcut did not work.
I often use the following shortcuts (among others):

CONTROL-x, . = · ('Greek full stop')
CONTROL-x, = (arrow right, Symbol font)

The keyboard as it were did not take notice of the CONTROL-x and
produced a full stop and greater than character instead.
Because my keyboard dated from te early bronze age, I replaced it with
a new keyboard, a Microsoft Comfort Curve keyboard with which I am
generally very pleased. But now this new keyboard shows the very same
misbehaviour, although somewhat less often. All other keys function
normally.
What (if anything) is wrong with the (left) function key?
I swapped keyboards only about ten days ago.

Kind regards,

Rob.