I just read online that there is a built in event log that can show you when you internet disconnects. Here is the forum where I read it.
internet connection status logging - TechSpot Troubleshooting
Here is how to get to the event log, but there is a lot to look through:
Event Log/Viewer errors/warnings:
1. Select Start
2. In the Start Search field type eventvwr and press the ENTER key
3. In the new window, expand Windows Logs and then select System -- this will display a list of all system events. Look for any errors and/or warnings (denoted by a red 'x' or a yellow '!').
*Note: You'll mainly be looking for DHCP and TCP/IP errors/warnings