Unable To Identify

I am running a website with a huge traffic on a single server with 2GB Ram and its working fine from last 3 years but from last 1 week the website is being unstable, not all the time but sometimes,

Following are the conditions when the problem occures:-

- When we enter the URL in browser and hit enter, it takes bit long on connecting...before reaching to the website, once the title appear at the top of the browser the page loads normally and while browsing the website, sometimes it gives page cannot be displayed error and we have to refresh the page.

- at that time when I check the result from [url]it gives bit high Average Response Time.

- On the other everything seems to be normal on SSH,
load average: 0.90, 0.96, 0.79
Mem: 3116360k total, 2841948k used, 274412k free,

root@silver [~]# netstat -alpn | grep :80 | awk '{print $4}' |awk -F: '{print $(NF-1)}' |sort |uniq -c
1 0.0.0.0
1197 ***.101.37.**

This is the normal traffic, not the attack and site was working fine from 3yrs but I guess some recently made changes is causing this issue.