Recent Problem - Losing Variables

We have recently started experiencing a problem with our ASP application (since around 27 August)

The problem is that we are occasionally losing our Session variables. On some occasions we are also losing variables in general (objects.) IIS is reporting that the variables are Null.

The symptoms seem to imply that the server is recycling or we are running on a server farm and the session variables are getting lost when the application switches to a load balancing server.

As mentioned, this has only started happening recently and no changes have been made to the application. Has anybody else started experiencing these types of errors?