Issue with select Comcast DNS Servers Only

Hello, I recently had a glitch where I renewed my domain and it reverted DNS settings. I've corrected the changes a while back and everything is fine except I cannot get valid DNS responses from 75.75.75.75 and 75.75.76.76.

All other DNS services work great (google dns, opendns, freedns, centurylink, etc). Even comcast's other dns servers resolve our sites such as 68.87.85.98 and 68.87.64.146. 

Is there anything that can be done to speed this up on comcasts end?

Thanks!