Don't reuse the Transformer unless you want to reuse the resources it has acquired. Cache the Templates object (the compiled stylesheet), and create a new Transformer for each transformation.
I am building an application that relies heavily on XSL transformations. In order to avoid repetitive XSL loading/compiling and document fetching (within the XSL), I have created a class that encapsulates the Transformer object, creating it at instantiation and holding it until the application shuts down. Although the XSL itself is not particularly big, it fetches a large XML document (using document() function) which takes time. Therefore, caching is essential.