Why has my Magento site become extremely slow?

Hi,

I have a Magento site and its traffic is high. Lately, the site has been becoming slow approximately once a day. About a month ago, it used to become slow about one time every 3 weeks.

As soon as I restart the Apache web server, the site becomes fast again.

Do you think that this is a bandwidth issue? If I increase the bandwidth on my server, will I solve this problem?

Thanks in advance.