Experiencing intermittent high latency, packet loss and general instability

We have had Xfinity for quite some time for our on-line business without much issue. Over the last couple of days we have found that any devices connected to via wifi go through large packet loss and high latency/ping. Running a speed test shows that, for the majority of the time, internet speeds are in the general area of our plan. 

If we connect our computers directly to the modem through ethernet cable everything seems to work fine. As soon as anything is moved to any of our wifi networks we are experiencing constant drops. I have done the obvious potential fixes (reset the router, reset the modem, purchase a new router, new cables to the router, etc.) and nothing has addressed the issue. If this coincided with the installation of new equipment or a switching of plans it would be easier to troubleshoot. Given that it just seems to have occurred spontaneously I am running out ideas.