We have a lab of Macs that this was happening to. It took us a while to work out the issue but it seems that there is a problem with the USB when you plug it in it doesn't connect properly to the port. we solved this by using the extension cables we got with the macs to connect to the keyboard and then the computer. they had a better connection.
Yup! @Luket..
Same issue, Same resolution.
It's a power issue, since attaching the extension cables, all is good...
And, That'll teach ya if you threw them away thinking they were of no use..
:_))
Thanks Guys I will check to see if I can find the cables and test.
This was the same for one of our sites. Only on Mac Pros and the extension cables worked around the problem.