Q: Why does my new iMac (27in, OS X Yos) keep randomly disconnecting from the internet?

My iMac randomly disconnects from the internet.  It is connected wirelessly to a Zyxel router (VMG8924-B10A). When the disconnection occurs the machine appears to be still connected to the router (ie the wireless connection symbol remains on) but there is no internet connection.  Other devices on the network are unaffected,though on occasion I think the iMac may also cause the router to crash.  Any help/advice much appreciated.