Trouble is that the equipment you're talking about would need to be authorized in order to communicate on the Cox network and run diagnostics. What are the chances Cox would allow random third parties to use diagnostic equipment on their network?
But the good news is your modem has diagnostic equipment built in. Look at the signals, errors, and logs. Especially look for how many uncorrectable codewords you have on downstream. Regardless of signal levels this should be zero or very close to zero. Look in the modem logs for T3 or T4 timeouts. You can also check CM-STATUS codes but this is a more advanced topic.
If you're seeing lots of uncorrectable codewords or if you're seeing T3/T4 timeouts, the problem is in your modem (very rare but it can happen) or somewhere on the cable system between your modem and Cox. If you've changed the interior cable with brand new high quality RG6, tightened all connections, and removed all unnecessary splitters, then it's probably a Cox problem. Call for a tech and ignore any threats about having to pay $75 since you know your equipment is good. Besides, you were willing to pay a third party for diagnostics and I guarantee that would cost a lot more than $75.
If you're not seeing problems with signals, errors, or logs then the problem is in your equipment. For example, even though your router has a gigabit port it might not be able to provide full gigabit speeds. This is especially true if you turn on security features like intrusion detection and antivirus. For example, some routers can give 940/35 speeds with intrusion detection and antivirus turned off. But if you turn on intrusion detection and antivirus the speed could drop as low as 300/35 because of all the extra work the router has to do to scan every packet. And of course all bets are off when it comes to wireless. There are countless variables when it comes to wireless that are outside of Cox's control.
I'm working on putting all of this information into a troubleshooting guide but I don't know when it will be finished.
