I think your claim is not true.
Let $H$ be the hyperplane section of $X$ and assume that $C$ is not linearly equivalent to a multiple of $H$. Then $h^0(X, C+rH)\to\infty$ for $r\to\infty$ (Rieman-Roch + Serre duality). On the other hand, $C+rH$ is not linearly equivalent to a multiple of $H$, so it is not a complete intersection.