Resolve Internal Server Error - Zen Cart / PHP / mySQL / Database

I need help resolving an internal server error which is affecting two of my websites which run on Zen Cart. I have attached a screenshot of the error. I have spoken with my hosting provider and I have been informed that it has something to do with the virtual memory usage that my site is using. I can see that by looking at resource details in the cPanel that this is maxing out sometimes. They told me to optimise the database (mySQL). I did this in MySQL using the automatic table update but they also told me that it needs manually optimisation. Basically I need an expert to fix this. And monitor it afterwards to ensure that the issue goes away. I have two website running of this shared hosting package and both websites are experiencing similar issues. You will need to resolve for both. This will be a simple project for an expert.