Sinusoidal wave output in single-frequency mode, sinusoidal instability

I have some PLL issues with the AD9914. I used a 50M external reference clock, N=25, and finally got a 2.5G system clock. By programming, a single-frequency mode is used to generate a 400M sine wave. But soon, the waveform will be distorted. I think it may be a problem with the PLL losing lock. How to solve this problem?
Thank you, look forward to your reply.