Boot up mac, (macbook pro 2011 yosemite 10.5.5) and all will works fine.

At some variable time, up to a couple of hours later, the network traffic stops, when I look at any of the following Outlook, Excel and Powerpoint they have stopped responding, close them down and 30 seconds later the network connection works again. 

All other apps during this process work fine other than they lose networking connectivity. 

Anyone any thoughts?