Comcast / Level3 Peering Issues?

Hey guys, VoIP provider here with primary connectivity through Level3 in LAX. For the past week+ we have seen dropped RTP packets between our Level3 connection and Comcast fiber customers located in Denver and Detroit Initially we worked under the assumption that something was occuring at the customer locations (such as localized packet loss), however we now suspect dropped packets between L3 and Comcast. Forcing traffic over to other carriers through community strings resulted in no packet loss. The issue appears to be in the evening and I was hoping either someone from L3 network ops or a list member in the know might have some useful information. Thanks, David