Web api stops after some time but works when I republish it but then again it stops after some time

I am experiencing a weird issue with web api on VS2013. When I publish the api in local IIS, it works perfectly but stops working after some time and throws this error

Could not load file or assembly 'Reporting.Models, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. Access is denied.

I have checked Reporting.Models DLL, it exists but the strange thing is if I republish, the api works perfectly but again after sometime it complains the same error.

Any idea why this is happening?