BTW are you using a router ? If so, try temporarily removing it from the mix and connect the Mac directly to the moden and see if the issue still exists. You'll need to powercycle the modem when you do this and then again when you re-insert it. The router may be acting as a DNS proxy and some have issues doing this.
Have used get dns automatically from isp settings and also, use these ip settings entering comcast dns 126.96.36.199 , 188.8.131.52 numbers manually into wireless router firmware up to date and correct version
wireless taken out of configuration, ethernet port uses different comcast dns numbers so to compare apples to apples manually entered in 184.108.40.206 comcast dns ...... minimum lag time and no timeout using ethernet port connected directly to docsis3 modem
test with wireless back in configuration
traceroute to 220.127.116.11 (18.104.22.168), 64 hops max, 40 byte packets 1 removed ip 14.133 ms 8.247 ms 2.502 ms 2 22.214.171.124 (126.96.36.199) 11.455 ms 11.829 ms 8.005 ms 3 ge-3-37-ur01.*******.ct.hartford.comcast.net (188.8.131.52) 7.799 ms 12.109 ms 11.014 ms 4 te-0-10-0-2-ar01.chartford.ct.hartford.comcast.net (184.108.40.206) 16.654 ms 14.751 ms 35.784 ms 5 pos-1-5-0-0-ar01.needham.ma.boston.comcast.net (220.127.116.11) 16.295 ms 15.104 ms 15.512 ms 6 te-4-3-ur01.chelmsfdrdc2.ma.boston.comcast.net (18.104.22.168) 21.012 ms 19.897 ms 19.423 ms 7 * cdns01.comcast.net (22.214.171.124) 18.489 ms 18.688 ms new test wireless router in configuration no timeout today will try later
one last configuration log........ Upstream channnle bonding value Channel ID 2 Frequency 23600000 Hz Ranging Service ID 6692 Symbol Rate 5.120 Msym/sec Power Level 39 dBmV Upstream Modulation  QPSK  64QAM Ranging Status Success
Downstream Bonding Channel Value Channel ID 8 Frequency 507000000 Hz Signal to Noise Ratio 37 dB Downstream Modulation QAM256 Power Level The Downstream Power Level reading is a snapshot taken at the time this page was requested. Please Reload/Refresh this Page for a new reading 14 dBmV when using a attenuator knocks that dbmv level down to 11
Using terminal and snow leopard os command for clearing out DNS
dscacheutil -flushcache fixes the problem temporally. When wireless is in the configuration, net monitor see repeated requests to dns just before a timeout . So either its my wireless or the way the dns requests are handled between comcast docsis 3 modem and my wireless .
We are not analyzing traces here. I only asked the poster to supply one to their default DNS server so that a CC employee like jlivingood or ctg1701a could see which DNS server they were being directed to as this originally appeared to be a DNS issue. I've seen this issue before with their new DNSsec servers and these guys ask for a trace to them..
Earlier in post had mentioned newest firmware and correct version ,but thanks for input. Thanks for the help EG. Am going to use the old comcast dns #'s, that hard wireing to ethernet port uses see if that makes a difference .
Entering manually older comcast dns #s 126.96.36.199,68,87,73,246 along with a change to firefox proxy settings from User system proxy settings to No proxy has made a difference to speed of loading pages both in text page and pics in craigslist .
On further testing last post did not fix timeout . The only thing that seems to help is clearing out cache in browser and using terminal command after each restart of computer and before visit to craigslist.