@TerryT 

For your situation, you would first want to log into the rotuer then go to:  Sytem Tools > Diagnostics and run a Ping Test using the following information:

IP Address/ Domain Name:  yahoo.com

Ping Count:  50

Ping Packet Size: 500

Ping Time out 800

Run the test 3 times and record the results.  If you see any packet loss above 3-5% the issue would likely be with the ISP or its service.  This is because if you run a loaded ping test from a router connected dirrectly to your modem this would be like running the same test from the modem to the computer it is eliminating any network device interference, range or load issues.  

If the pings are clean the next thing you want to make sure is that your modem is just a modem and not a gateway device.  If it is a gateway and its operating as one then you are in a double NAT enviorment which can affect network performance.  In this case you would want to contact the ISP and put the modem/gateway in modem only mode.  

If you are only in a modem enviroment and the ping test is clean, the last thing you want to make sure is your firmware is up to date, if it is a replacement may be needed if you are still within warranty.