The slow speeds thru the 68U are very strange to say the least. There haven't been any changes to the modem firmware for several weeks now so I wouldn't expect any issues from the modem either. Do you happen to have IPV6 enabled on the 68U?
1. Have a look at the connected port LED at the back of the modem. It should be amber in colour, denoting a 1 Gb/s connection with the 68U. If its green, that denotes a 10/100 mb/s connection. If you have slow data rates thru the 68U thats the first item to look at. Swap the ethernet cable with one that you know is a good cable.
2. The NAT Acceleration should be enabled for speeds over 100 Mb/s. The problem with NAT Acceleration is that various functions can disable it. Those include Traditional QOS, any form of traffic monitoring and a couple of others that don't come to mind at the moment. End result, your throughput rate drops. What I would suggest is to run a factory reset on the 68U, then after the reset is complete, go thru all of the menu items on the left side of the settings menu, and all of the sub-items. Disable everything except for AI protection, the firewall, NAT Acceleration and wifi. Essentially, get the 68U to the point where you have a minimal set of functions running. At that point you should be seeing something like 328/22 on a speedtest if you use the www.speedtest.net Toronto Rogers or Beanfield server or Montreal Rogers or Fibrenoire server. At that point, if you decide to run other functions, enable them one by one, and after each function is enabled, check the NAT Acceleration to ensure that the selected function has not disabled it without any warning and run a speedtest to determine what the effect is on the 68U throughput. In cases such as a Samba server, you would have to have that function in use when you run the speedtest. Running a speedtest with the function enabled but not necessarily in use probably wouldn't show a true result.
Having said all of of that, if for example, without doing anything, you see a speedtest result of ~328/22 during the day, and at other times of the day the download drops to 5 Mb/s for example, my first thought would be an issue with node congestion. In that case it would be time to look at the latency to the CMTS, which your modem is connected to. That can be determined by downloading and running Pingplotter, which will run in freebie Pro mode for 14 days before it drops down to freebie mode. You can decide to buy a standard licence (cheaper) or the Pro licence (not so cheap) if you decide. Have a look at the following post on setting Pingplotter up to monitor the modem to CMTS path:
Although that post is aimed at using Pingplotter for packet loss issues, running Pingplotter for 2 to 3 days continuously will illustrate if there is a large increase in the latency to the CMTS during evening hours. Set pingplotter to run at 1 second intervals and just let it run.
Edit: just looking at another post and wondering if the modem's RG-6 cable is connected directly to the wallplate's cable port or if it is running thru any type of surge arrestor that is typically found on a power bar?
Yes, I enable the IPv6 because I need to use some IPv6 websites. The Cut Through Forwarding is enabled. My problem is usually I can get the 300Mbps internet, but after a few days or a few hours, the speed suddenly drops dramatically to 4~6Mbps by the wired and wireless from the asus router.
With IPV6 enabled, you will see a huge drop in throughput speeds if AI protection is enabled and the data source happens to be IPV6. Just to illustrate that, try the xfinity speedtest with and without AI protection enabled. That should illustrate the throughput difference as its tests both IPV4 and IPV6. Select the Detroit and Chicago servers to run the test:
@gp-se had a good idea when the slow speeds occur, and that is to disconnect the PC from the 68U and connect directly to the modem. Don't do anything to the modem or 68U. Leave them running as is. The question at this point is, is this a modem issue or a 68U issue. Run a speedtest to see what the throughput is with a direct connection and then reconnect back to the 68U.
You could also run IPV4 only for a two to three day test to see if the same slow data rate situation occurs. That might be a pain with web site access, but, it would point out whether or not this is IPV6 related.
Thank you for your help, I already disabled the AI protection, and when the slow speed occur, If I connect the computer direct to modem, I can get the 300Mbps, then I connect computer to router again, I only get 4~6Mbps, but at that time point, if I change the port which router plugin to the modem, the computer which behind the router could get speed grow up to 300Mbps. So I guess if there is something conflict between the modem and the AC68U?
It is amber. And I read some posts said that in the front panel, there is a light looks like a pc should flash blue, but in my modem it is green, is that ok?
@AlbertJiang the only thing that comes to mind is the possibility of a port controller failing, on either the modem or 68U. For the 68U you might be able to prove or disprove this by kicking the modem into Gateway mode and switching the 68U into Access Point mode. Let the modem run the network for a couple of days and see if you have the same issue. Run the connecting cable from the modem's LAN port to one of the 68U's lan port to bypass the 68U's WAN port. If you see the same failure, the next step would be to swap out the modem. I suspect that you might end up with a CODA-4582 which is a work in progress, however, I run that modem in Bridge mode with my 68U in full router mode and haven't had any issues with either one. It seems that customers who use the modem in Gateway mode are experiencing issues with it, but in Bridge mode, I think that I've bypassed those problems.