Contact Thomson either directly or through a local distributor for assessment. Odds are it will be covered under warranty or they can tell you what the deal is.

Even high end zoot stems can fail. Stems are dicey - always. I haen't really met a stem I would trust blindly since threadless came out...honestly....

Also...as for checking for cracks around your whole bike. I have been recommending that everyone should do an annual teardown for just such a reason. It also helps you identify components that may not be functioning properly anymore, adjusted incorrectly, or near failure or worn.