Proble with NCO core at frequency higher than 200MHz

Hi All

I am using Arria V FPGA. I am using NCO ipcore for waveform generation as well as carrier. The problem I am facing is with NCO core. At higher frequency , lets say 240 MHz, if I am generating 30 MHz, which is integral multiple of 240Mhz, spectrum is clean. If I move to 31 MHz,spectum becomes noisy. Any solution.