WordPress site ultra fast, then hours of slowness

Hi,

I’ve been running a wordpress site (www.barbicanwaterfront.com) for 2-3 months now and have had good level speed from the site.

Until now :(. On Saturday my site in the morning was like normal, I installed 2 plugin updates and posted a news article, then I noticed my site pages taking ages to load including pages within the dashboard. Error 500 appear a few times(this might of been due to the slowness?) I tried various things with no luck. This occured from 12pm (GMT) to late evening, where it improved slightly after I disabled all plugins and removed any that where not nesscary to lower the amount used on the site.

Thinking it was my hosting (godaddy) having a moment or a plugin conflict I managed to remove I waited until the morning, the website was running at a reasonable speed at this point.

Sunday: morning website was good as new, fresh and fast from 8am (GMT) to around 2pm (GMT) when it slowed to a crawl taking 8-10 seconds to load a page.

I disabled all plugins and renabled them slowly. W3 Total Cache appeared to add 2-3 seconds page on loading so I had to disable it. Each page took 8-10 seconds to load (was consistant on all pages). I gave up and left it. I noticed the site speed up for a few minutes at times then slow to a crawl again throughout the day. I left it, then at around 8.30pm the site speeds up! pages taking around 3-4 seconds to load.

Monday: Website is lightning fast pages taking 1-2 seconds to load everything appears fine. I post 2 news posts and the site gets very sluggish slowing down until it’s taking ages to load each page :(.

Summary:

I think something is going on when I post something to the site, but not sure where to look. Any suggestions would be very welcome as I am at a total lost at this point :(.

I checked with godaddy who came up with: “Thank you for contacting online support. The domain is not connected to the host. It needs to be using our hosting nameservers and be pointing to the IP address of the hosting account. Below are the directions you need to follow to correct everything…. To modify the A-Record for a domain:
…”

The website was working fine for 2-3 months and I am not sure if playing with the hosting account settings will make the problem worse? Anyone know what an A-Record for a domain is? 🙁