Keyboard stops responding randomly

I have a 4 month old Viewsonice USB keyboard. Recently it has stopped
responding at random intervals, often several times per day. I have to
unplug it and reinsert the USB connector. Then it works OK until the next
time. Usually I have noticed that the numlock LED has gone out before it
locks up. I've tried a different USB port but that didn't solve the
problem. Any suggestions?

Thanks.