Run-time error '3085' - Undefined Function in expression

I have an intermittent problem with an Access database. I have a function defined in a module shiftletter, which takes two parameters. This function is used by a query. Nine times out of ten the query called by VBA works. But occasionally I get the above error, to make the query work I have to reboot my machine. Opening and closing the database does not help. Running the same query in the query window works.

I have tried moving the function to a new module; compressing the database and using the repair option. None of these have cured the fault.

Anyone got any ideas