An analysis is presented for the class-M9.3 solar flare of November 6, 2004, whose decay phase displayed weakly damped harmonic oscillations of the predominantly thermal X-ray flux detected by the RHESSI spacecraft (at energies ≲25 keV). The period of these oscillations was ≈78 s, and their characteristic decay time ≈100 min. Similar quasi-periodic pulsations were observed in the decimeter-centimeter radio flux (pulsations of a type-IV radio outburst), but were less pronounced in the non-thermal hard X-ray flux (≳25 keV). The area of the quasi-stationary X-ray source, which was located primarily at the apex of a set of flare loops (≲15 keV) that were cooled primarily via thermal conduction, was found to be in anti-phase with the oscillating X-ray flux it emitted. The observed oscillations are interpreted as harmonic modulations of the radiation flux emitted by the heated thermal flare-loop plasma, due to the global, standing, sausage mode of fast magnetoacoustic waves excited in the loop.