Intermittent problems with cut/copy and paste (using CUA keys) between Emacs and other programs

[nljlistbox2]

I'm having occasional problems with the clipboard / kill ring (?) and I am 
wondering how to go about narrowing the problem down sufficiently to report a 
bug.

I have GNU Emacs 24.3.1 (x86_64-redhat-linux-gnu, GTK+ Version 3.8.2) (of 
2013-08-14 on buildvm-15.phx2.fedoraproject.org) on Fedora 19 GNU/Linux 
3.11.8-200.fc19.x86_64 (of Wed Nov 13 16:29:59 UTC 2013) and I have been 
suffering the misfortune of using Gnome 3. I use CUA keys for cut/copy/paste.

The problem is that occasionally when I try to paste text in to Emacs that I 
cut/copied from an another program (usually Firefox), either nothing happens or 
the text that gets pasted is something I cut/copied in Emacs at some point 
previously. Likewise, occasionally when I cut/copy in Emacs and paste in to 
another program, my text from Emacs doesn't get pasted. Most of the time, 
however, these operations work as expected.

That is, it seems as if the clipboard / kill ring inside Emacs sometimes gets 
dissociated from the system clipboard.

The problem occurs maybe once or twice a week and only (I believe) after I've 
had Emacs running for a few days, not when it was freshly started, but I am not 
100% certain that this statement is true. The problem has been ongoing for 
several months -- certainly since September, and I suspect it's been happening 
since I set up this computer in July.

After the problem occurs, sometimes (always?) if I repeat the operation using 
the Edit menu, the operation works, and thereafter the CUA keys start working 
again (for a while at least). 

One difficulty with observing what's going on and narrowing it down is that 
Ctrl-c, Alt-Tab, Ctrl-v is so much second nature that it happens below the 
level of consciousness, so when it fails I'm jarred out of my thought space and 
have no recollection of what specifically I just did. However, once the problem 
happens once, if I repeat the operation it almost always fails again, so at 
least I can confirm it really happened even if I can't identify anything I 
specifically did to make it happen in the first place.

Thanks in advance for any pointers anyone can provide.

N. Jackson.