TaxonomySession.DefaultKeywordsTermStore Intermittently Returns null

I have created a SharePoint 2010 timer job which utilises the SharePoint Managed Metadata Service.

The code I have is :

TaxonomySessionsession =newTaxonomySession(this.SharePointSite);

if(session ==null)

   LoggingService.LogVerbose("Taxonomy store unavailable");

   returnnull;

// Get the default keyword TermStore for the provided site

TermStoredefaultKeywordStore = session.DefaultKeywordsTermStore;

if(defaultKeywordStore ==null)

    LoggingService.LogVerbose("Default Keyword store unavailable for Taxonomy Session");

    returnnull;

From time to time,defaultKeywordStore is returned as null - occasionally, for periods, every defaultKeywordStore is returned as null for every call.

Anyone seen this sort of behaviour before?

Thanks for reading

returnnull;