Assembly caching question

[murphy]

During my use of vs.net for a web project I sometimes get the following
error.

******
Server Error in '/HD' Application.
Configuration Error
Description: An error occurred during the processing of a configuration
file required to service this request. Please review the specific error
details below and modify your configuration file appropriately.

Parser Error Message: File or assembly name HDr, or one of its
dependencies, was not found
*******

It occurs after I've built and run the code and then left for the day.
I return in the morning, needing to reset IIS, reload the solution and
rebuild the solution. Some portion of this routine makes the problem
go away. Of possible note is that HDr.dll is not built by the project.
I simply keep it in the target bin folder. I explored the temporary
asp.net cache and found no reference to HDr.dll after the error has
occurred. I guess the error means that it is expected to be there.

My question: I have a suspicion that that adding a reference to this
dll in my project will solve the problem by causing the dll to be
maintained in the cache. Is this correct and advised for any dll used
by the application? It's confusing because the problem is
intermittent.

Thanks!