This could be a problem with Comcast DNS if they are caching records longer than Google's advertised TTL. Using Comcast's DNS servers, can you run "nslookup" and do the following:
> set debug
> www.google.com
The output at the end should have something that looks like this:
QUESTIONS:
www.google.com, type = A, class = IN
ANSWERS:
-> www.google.com
canonical name = www.l.google.com
ttl = 82165 (22 hours 49 mins 25 secs)
-> www.l.google.com
internet address = 64.233.167.147
ttl = 238 (3 mins 58 secs)
-> www.l.google.com
internet address = 64.233.167.99
ttl = 238 (3 mins 58 secs)
-> www.l.google.com
internet address = 64.233.167.104
ttl = 238 (3 mins 58 secs)
What do you get back for TTL values when it's not working? Google has their www.l.google.com records set to a 300 second (5 min) time to live (ttl), so if you see a number bigger than that in the addresses that resolve for www.l.google.com you could very well have a DNS issue.
-Eric