Wireless network

I have a pc on the wirless net work that in most cases works fine while I am working on it, but if I leave it for some time and return I have to disable the network connection & re-enable it to connect back to the network… the icon in the bottom right hand corner is flashing saying connected then disconected… I have used the repair function this did not work .. the message from this was ( could not get IP address ) the strange thing about this question is….. sometimes it stays connected for 2-3 hours sometimes 15 mins.. please could you give any answers..??