Unstable cyclic modulation during the main outburst of KS 1731-260

► The cyclic X-ray modulation is transient and highly variable during the outburst. ► The cycle is not caused by transitions of the disk between the hot and cool state. ► We favor a disk precession with mode switching and unstable warps. ► Variable absorption of X-rays cannot be dominant in producing the modulation. ► Variations of the stream impact on the tilted and warped disk are promising.