GS108Ev3 IP address

I have seceral Netgear switches (GS108Ev3, GS105Ev2, JGS524Ev2), which all obtain IP addresses from the router (DHCP mode enabled). Despite DHCP mode enabled some of the switches switch from an IP address in my router's range (e.g. 192.168.178.xxx) to the IP address 192.168.0.239 which is not in my router's range. Even stranger is that several switches have the same IP address 192.168.0.239. As a result I cannot the switch's Admin web page since several switches have identical IP addresses. Restarting a switch (power off->power on again) results in an IP address in the range of my router but some time later it is back again to 192.168.0.239. What is going on here and how to prevent this from happening?

Leon