  Let $(M,g)$ be a compact riemannian manifold of dimension $n\geq 5$. We are
interested in the stability of a slighly subcritical Paneitz-Branson type
equation on $M$. Assuming that there exists a positive nondegenerate solution
of the critical equation and under suitable conditions, we prove that this
equation is not stable for all $n\geq 5$.
