Solution
= Solution
Linearization about $(0,\Omega,0)$ gives $\dot u=-Av$, $\dot v=-Bu$, where $A=(I_3-I_2)\Omega/I_1$, $B=(I_2-I_1)\Omega/I_3$. With $\kappa=\sqrt{AB}$,
$$
u=u_0\cosh\kappa t-(A/\kappa)v_0\sinh\kappa t,quad v=v_0\cosh\kappa t-(B/\kappa)u_0\sinh\kappa t.
$$
The exponential growth confirms intermediate-axis instability.