In this case we would suggest you to perform hard reset on the modem then check speed on direct connection and also slow speed, intermittent connection or no internet access occurs usually out of cable signal level, out of acceptable range and it will be resolved after the power level are adjusted by the service provider. So for this we need to check the cable signal level to confirm what causing this issue to proceed further.
Here we are checking the way the signal comes from out to inside the home. There are some important things that should be within a specific range in order for the internet to work properly.
First, the SNR should be above 33 on all of them. The downstream power should always be between -15 and +15. The closer to 0 the better connection. Finally, your upstream power should always be between +45 and +51
On all of those values you should not have more than 3 decibels (or dB) between the lowest and the highest
By having the values in less than that, your device will have interrupted connection, slow or no connection at all.
Please follow the steps mentioned on the below article to to check the signal level on the modem interface.
http://arris.force.com/consumers/articles/General_FAQs/SB8200-Cable-Signal-Levels
They were able to bring the levels to within range, but the overall speed remains low. Increased from 3-500 to 5-700mbps on a 1gb plan
Could you let us know whether the slow speed connection is with hardwired connection (directly connected to the modem) or with Wi-Fi access while connecting with router.
If you are experiencing the slow speed internet connection through hardwired connection which is the modem connected to computer through ethernet cable, I suggest you to try with
Re provisioning the modem with your internet service provider.
Also share the cable signal levels to know whether all the signals are in range.
