SharePoint returns 500 error message while calling the web service method asynchronously?

We have two custom web-services hosted on the SharePoint 2010, one of those is used to upload the document on the library(folder) and another one is used to update/add item on the list.

We have built the unit test cases(VS2013) which would call these web-services asynchronously(task based pattern).

If we execute the test cases, it works just fine for fewer iterations and started throwing errors(especially internal server 500 or unable upload the documents of 100kb).

I doubt if this is something to do with the server performance instead of an issue with the code, however kindly suggest.

Also, How could I handle this issue if this is more of a performance issue? How could I make sure that the task based pattern works in this scenario. Would highly appreciate if you could help me with a sample.