CTRL+C doesn't always copy into PC's memory.

Hi folks.
I have a rather weird problem.
I use CTRL+C / CTRL+V to quickly copy/paste (text) things from one window in
Vista to another window in Vista. For example: CTRL+C from part of text in
email and CTRL+V to IE window.
The issue is: sometimes CTRL+C doesn't work for me: text is not being
inserted into PC's memory - all I get when pressing CTRL+V in other window to
paste my selected text - I get either previous copied text or nothing from
PC's memory.
I get an impression that my PC doesn't keep up with my pressing CTRL+C, but
even if I press it slowly, several times, then sometimes I also get that
glitch with CTRL+V having old text or nothing at all.
Nothing special installed on PC, PC is running smoothly otherwise, no
sticky-keys. (Installed newest driver from Logitech - nothing changed).
Any help?