@Saav 

A few things to check:

• Power issues such as a UPS going bad or power brown outs/spikes.
• Heat issues where the router's air is blocked or clogged.
• A USB device that is pulling to much power such as a hard drive.
• The Ethernet cable is failing or the connector is bad or dirty either at the modem or the router.
• The connection to the modem is loose.
• The modem shows connection errors in its log.
• Recent firmware change.
• Recent device added.