WCF error: "The service cannot cache the negotiation state as the capacity '128' has been reached"

I'm trying to find out why one of my WCF services sometimes randomly makes the lsass process take 100% of the CPU. I was looking through the WCF trace logs and found this error many times:

<ExceptionString>Syst开发者_StackOverflow社区em.ServiceModel.QuotaExceededException: The service cannot cache the negotiation state as the capacity '128' has been reached. Retry the request.</ExceptionString>

The WCF service is hosted in IIS6.

I've searched online but can't find what this actually means, or where I can set the capacity, or what might be causing the cache to get full.

Any ideas?