I don't know if it's anything to do with ISP bandwidth and number of machines but 7 does seem to drop out quite a bit since installed on 3 machines and all the netbooks also suffer from this symptom. Apart from the list, open Network and Sharing Centre and you should see a 'red cross' between your network name (SSID) and the internet:
Option 1: Click on the cross to 'repair' the connection.
Option 2: Switch off the router for 20 seconds then back on again.