Strange mouse action in Gnome/Ubuntu

[Doubletwist]

Whever I click on anything, whether it be to open an application from 
the menu, or even clicking menu items in an application [such as 
Thunderbird], about 60% of the time, the application or item is opened 
twice, as if I clicked it twice. If I click and hold for half a second, 
it acts normally and opens once. But when I click like I normally do, 
which is fairly quickly, it opens twice [not always, but most of the 
time]. I know I'm not double-clicking, and I"ve even tried adjusting the 
double-click rate in the Mouse prefs. To no avail, the problem remains.
I don't know if this is a Gnome issue or an Ubuntu issue. I have always 
used KDE in the past [usually on Debian or slackware] and never had this 
problem before on this system with this mouse, nor does it happen in 
windows.

Any help, or pointers to help would be greatly appreciated.

Best Regards,
DT
aka Jim