no, its probably not your isp or router (well maybe it is, i'm dumb). go to the command prompt and type ipconfig /all . in the list of your network devices find the adapter that's connected and look to see when your 'lease expires'. if the difference between 'lease obtained' and 'lease expires' is about the couple of hours that you mention, then your dhcp lease isn't long enough. unforunately, i haven't got a clue how to fix this... try googling or looking in your router settings.

hope that helps

*edit* to clear things up, the dhcp lease is the time the router allows you to have your ip address. there are several ways of renewing your ip. you can either restart your pc, like you said, or you can type ipconfig /renew to renew your dhcp ip address