DoS Attack ???

I was experiencing random dropoffs on some websites only. The connection would be up and fine and I can browse and everything but some websites woudl just stop working. I started looking into it and noticed that exact at that time these DoS Attack messages were there in the router log.

Its a NG C7000 V2. and the dos attack prevention option is On. 

Is this something to be concerned about ? Is that 75.75.75..... ip the comcast dns ? whats goin on here ? Please advise.

[DoS attack: TCP- or UDP-based Port Scan] from 75.75.75.75, port 53

DoS attack: TCP- or UDP-based Port Scan] from 75.75.76.76, port 53

[DoS attack: TCP- or UDP-based Port Scan] from 75.75.75.75, port 53

[DoS attack: SYN Flood] from 13.68.247.210, port 443

[DoS attack: SYN Flood] from 35.166.131.228, port 443

[DoS attack: SYN Flood] from 44.233.3.2, port 443

[DoS attack: SYN Flood] from 151.101.42.49, port 443

[DoS attack: SYN Flood] from 104.214.58.194, port 443