Slow mySQL queries causing high resource usage on server

Hello,

I have an issue with one of my sites. The domain is bambolino.gr. I got too frequently server / service unavailable errors and after i contact my host provider found that this is causing due to some slow database queries.

I want to know if there is any plugin or another way that you suggest to find out what exactly plugin/script causing this issue.

Any suggestions appreciated.

Best Regards,
Kyriakos