Mouse doesn't respond after sleep

Hi everyone,
this is my first post here and I'm really happy with my PowerMac which I haven't even had for a week yet.
There is one thing that I find a bit strange though - sometimes when my computer wakes up from sleep mode, my mouse doesn't work. In order to get it to work, I need to unplug it from the keyboard and repeatedly plug it into the usb port on the front several times. Once it works, I plug it back into the keyboard port and it works perfectly.
Is the keyboard usb port or the mouse faulty? If not, would plugging it into the usb port on the tower itself solve the problem?
Thanks for your help