We recently upgraded our service package to 600Mb from 200Mb and have seen no increase in speeds. In fact, and maybe just because I'm looking more, speeds are worse/all over the place, ranging from 90Mb to 250Mb, occassionally hitting close to 300Mb. Also having some latency issues. When we upgraded, I realized our modem was not up to the task of transmitting 600Mb (rated for 343Mb), so I got a new Morotola MB7621 (Gigabit rated), which is a Comcast certified modem. I've talked to Comcast support a couple times, and they said things look Ok on their end, but I was just checking in the modem interface and have found some errors that I've seen others reporting here. I'll post the errors and connection stats below. The previous owners on the house had cable running into nearly every room of the house and had a 5 way splitter installed in the junction box, which I have removed so it's just the incoming line running directly into the modem. No cable TV, no splits, although there is a junction from the line running into the house into an inside cable.
Are there more error log entries being shown ? If so, please post them in their entirety. Those "Time Not Established" ones that you posted indicate only one single event.