I am in charge of a website that has about 150 unique users a day. The website hosts forms and gauge data, providing calculations and alerts. It runs well, but for some reason there are intermittent long response times; even for simple requests. For instance I will load one webpage in less than 2 seconds and then another time it will take 20+ seconds to respond. I've even had it take up to 60 seconds before. 3/4 of the time the pages will load fast, but 1/4 of the time it takes much longer to load the same exact same content. I used htop during the long response times to see if there was a program hogging all the CPU or memory, but I could not get consistent results. Sometimes the server load was high when I got a slow response and sometimes there was nothing running at all when I got a slow response time.
I am new to system administration so I'm struggling to find a good way to pinpoint the error. I've checked all of my logs and found nothing.
Here's a screenshot from atop when a request was taking a while to load: http://imgur.com/WFJIME1
and here's a screenshot from htop: http://imgur.com/rMUxVZ1