Can you try bypassing the router and connecting the problematic computer directly to the modem to see if the issues still occur?
Technical support needs your feedback. Please take the time to fill out a survey using the link below and tell me how I am doing:
https://www.surveymk.com/s.aspx?sm=aW04S5FAdACKYEK_2f1kLKxQ_3d_3d