VoIP quality reduction

I find it interesting to note that my VoIP quality of the two services (Vonage and Sun Rocket) I utilize for my consulting business have degraded since Comcast(my ISP) went into the VoIP business. I utilize their premium service with 8 mps down and 768 kps up. This should be more than adequate for their claimed 90 kps requirement. Maybe it is my perception and then again, maybe not.