show your components.xml snippet to configure the SMPC, e.g.:
<persistence:managed-persistence-context name="entityManager" auto-create="true" persistence-unit-jndi-name="java:/fooEntityManagerFactory"/>
Intermittently null sounds very buggy/sketchy to me. You can't configure the SMPC to be sometimes null when it's injected into a Seam component (before every business method call).
Plus I would upgrade to 2.1.2.GA if I were you...
I've never heard of this happening before, and it's a major problem even if it happens just once per session (I've used Seam since 1.2.x).
I've been using RF 3.3.1.GA for a month or two now, you can most likely rule those libraries as the root cause.
Sorry, I meant rule out