C7000 with Comcast Infinity super slow speeds

My router/modem has recently started having connectivity issues or super slow/ non responsive internet connectivity.  I have Comcast Infinity Blast since January of 2016.  When the service is working properly I am getting very fast speeds (75-90 Mbps), however lately my speeds have been dropping to less than 10 Mbps and more often than not less than 1 Mbps.  After restring the router speeds will increase to normal speeds but drop back down within 15-20 mins.  Comcast says that the problem is with my equipment and that they can not support any troubleshooting with my customer owned equipment.  Is there anyone has had similar issues that could offer some support to help resolve this problem?