I have several of them in use and have been very satisfied so far, but now one has a very strange behaviour: It is "off" even though the power cable is plugged in, the switch is on and there is power on the socket. When I turn the switch off, I hear a low but very high-pitched whistle. When I then switch it on, all the lights flash briefly, then it is "off" again. If I do this several times, it runs for about three to five minutes the third time and is then "off" again. Has anyone had similar experiences and what could be the cause?