looks like peak time congestion from what you have described. to confirm, you could post a pingtest.net result
also, to make sure, you could post your modem stats from
http://192.168.0.1 using the default login of
admin and password
changeme then click on
advanced settings under the three big red buttons and click on
connection about half way down the page on the left. Once your there just copy and paste all the stats