CoderXO - Internal Server Error 500 - Joomla / Virtuemart

We get intermittent Error 500 Internal Server Errors in our Joomla / Virtuemart. It happens mostly in the administrator backend, but also occassionly in the front end. It happens mostly when performing a search e.g. search for a particular product. But it can also happen by just clicking any link. See example of error below: Error 500 Internal Server Error Your browser sent a request that this server could not understand: /administrator/[login to view URL] The request could not be understood by the server due to malformed syntax. The client should not repeat the request without modifications. NB - Please NOTE: It is important that should you make any chances / Repair / Optimise the database that you first perform a backup. It has happened before where a repair on a table / database caused us to loose 1000's of products and categories. If this happens we need to be able to determine which products / categories are missing so that we can add them again. You will need to assist with a query or some mechanism to quickly pull a list of missing products / categories (Product Name, Product SKU and related categories.)