Ajax Request Returns Internal Server Error

I have a drop down list which is populated with an ajax request, that is called with selecting a value from another drop down. The request is handled perfectly well every time, but when no operation is done on the page for a while like 2 or 3 mins the request returns an internal server error. For example:

Ajax Request = Successful
Ajax Request = Successful
Ajax Request = Successful
-Wait on the page like 2 or 3 mins
Ajax Request = Internal Server Error - 500
Ajax Request = Successful
Ajax Request = Successful
Ajax Request = Successful

What is the cause of this, can you help? The weird thing is when request is repeated after the server error it re开发者_StackOverflow社区turns the result succesfully :s

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