Seems odd, booting modem/router should not make your DNS problem better. click up a CMD terminal and do a ipconfig /all
When you have a outage you could ping your gateway or/and ping 66.94.234.13 to test net speed/access to yahoo.com ping yahoo.com to test dns lookup and get speed
Did they tell you about the service call if no problem found?
You could try looking at the modem web page at http://192.168.100.1 to check up/down stream signals
before, during and after an outage. ping 66.94.234.13 to verify connectivity. nslookup yahoo.com to verify dns
Note, windows can cache the lookup so the second lookup could work with dns out.
What you do is lookup different sites you have not been to everytime google.com, msn.com, yahoo.com, tv/radio stations,....
It might, comcast dns lookup time dig @63.240.76.198 yahoo.com | grep "Query time:" ;; Query time: 136 msec
speakeasy.net dns lookup time
dig @66.93.87.2 yahoo.com | grep "Query time:" ;; Query time: 71 msec
Of course it would need to be the first dns server in your list.
You would have to run tracert to each server to see time to server for that point in time. Route could be slower later in the day. :(