Assistance with sorting performance / security issues on Wordpress sites

General information for the website: WooCommerce site selling wood fuel - lcenergy.co.uk
Kind of development: Customization of existing website
Description of every page/module: We need some assistance with a hosting / wordpress issue that is causing our server to run out of resources on a regular basis. The server is hosted with Site5 who have not given much help in de-bugging this issue. The server runs about 40 wordpress sites and 2 joomla sites. Three of sites get regular and reasonable traffic, two are WooCommerce sites.
Site5 think there is a Cron issues or conflict. We are not so certain.
Description of requirements/features: We are looking for someone to take a look at the server and sites and try and find the problem. I guess it could be a hack or conflict or virus.

Looking for guidance and offers and suggestions.
Extra services needed: Optimization
Extra notes: