Your ISP is using Cogent as their Tier 1 transport and the problem is with the gateway between AT&T and Cogent.
Unfortunately, the Internet is not something we have direct control over. You need to let your ISP know about the problem.
I'll also let AT&T know, but Cogent is responsible for that gateway.
Wonder what the trace looks like at a different time? My guess is the gateway is being taxed. If so, it should look better at non-prime times. If the issue is not related to a router being taxed, then the problem should persist, regardless of the time of the trace/Ping Plot.