We report and analyze oscillatory behavior of lineshapes in two-dimensional photon-echo relaxation spectra of a perylene-based dye molecule, whose four-wave-mixing signals are strongly modulated by coupling to low-frequency vibrational modes. Vibrational wavepacket motion is found to induce a pronounced beating of the anti-diagonal absorptive peak width, accompanied by orientational changes in the dispersive signal part. The effects are reproduced well by simulations based on a Brownian oscillator model, and can be assigned to periodic alternations in the relative amplitudes of rephasing and non-rephasing contributions to the spectrum. (C) 2008 Elsevier B.V. All rights reserved.