I have a solution to this problem.
I had exactly the same problem. Just popped up out of the blue a few weeks back -- my internet connection would go down about every 20 minutes, some times more frequently, somethimes less. A sample of my error log is below:
2008-07-02 12:22:45 3-Critical 0x040D9A2C DHCP WARNING - Non-critical field invalid in response.
2008-07-02 12:22:42 3-Critical 0x040D9964 DHCP FAILED - Discover sent, no offer received
2008-07-02 12:22:31 3-Critical 0x04E33948 No Ranging Response received - T3 time-out
2008-07-02 12:22:13 3-Critical 0x04E33A10 Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 timeout
2008-07-02 12:21:41 3-Critical 0x04E33A74 Started Unicast Maintenance Ranging - No Response received - T3 time-out
2008-07-02 12:18:21 3-Critical 0x040D9A2C DHCP WARNING - Non-critical field invalid in response.
After much thought I did three things:
[1] I loosened and retightened all my F connectors. At the drop outside and at the two way splitter that takes one leg to the cable modem and the other leg to my 4-way amplified splitter for my TV's.
Result = NG
[2] Next I made up a new RU6 cable to run from the modem to the splitter thinking the old cable might be shot.
Result = NG
[3] I REPLACED THE TWO WAY SPLITTER
Result = NO MORE PROBLEMS
Apparently these little puupies can and do go bad. In addition, I have seen where cable techs recommend not using gold plated splitters -- my old one was gold plated made by GE.
Problem solved and no a single friggin' error in the past 48 hours. Incidentally my modem is a 2002 SB5100 and my service is Optimum Online in Morris County NJ. Also running through an AT&T VOIP ATA and a Linksys wireless router.