IIS7 ISAPIModule Intermittent Error 500 - Incorrect Function

I'm attempting to diagnose an intermittent failure of a web application. I've enabled failed request tracing, and have collected logs. The error I'm encountering is:

ModuleName="IsapiModule", Notification="EXECUTE_REQUEST_HANDLER", HttpStatus="500", HttpReason="Internal Server Error", HttpSubStatus="0", ErrorCode="Incorrect function. (0x80070001)", ConfigExceptionInfo=""

Some general info: The application is ASP.Net 4.0, and it has it's own dedicated app-pool (runs on 4.0, Classic mode).

I'm at somewhat of a loss in terms of where to turn next. The information provided by the trace doesn't seem to specify anything more specific. Suggestions welcome!