USB Keyboard works occasionally

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http://askubuntu.com – A few days ago I bought a SL640 Hama USB keyboard to use on my laptop with Ubuntu 12.04. But I'm having problems, as it works one time out of 10! While on my girlfriend's laptop, with Win Vista, always works. And on my laptop I have Win7 in dual boot. And I have to say that it always works on it. Here are some infos: ~$ lsusb Bus 006 Device 003: ID 04d9:1503 Holtek Semiconductor, Inc. Shortboard Lefty As you can see, the OS recognized it but the keyboard doesn't react ... even the Caps Lock and Num Lock keys don't blink. Regarding the legacy support (useful for GRUB), I have found no entry i (HowTos)