We study statistical properties of the $0^+$ spectrum of $^{208}$Pb in the energy region $E_xleq20$ MeV. We use the Skyrme interaction SLy4 as our model Hamiltonian to create a single-particle spectrum and to analyze excited states. The finite-rank separable approximation for the particle-hole interaction enables us to perform the calculations in large configuration spaces. We show that while the position of the monopole resonance centroid is determined by one phonon excitations of $0^+$, the phonon-phonon coupling is crucial for the description of a strength distribution of the $0^+$ spectrum. In fact, this coupling has an impact on the spectral rigidity $Delta_3(L)$ which is shifted towards the random matrix limit of the Gaussian orthogonal ensembles.
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