plutoSDR sampling rate > 4Mhz issue

I use grc to build a GFSK mod/demod flowgraph .

my setting is , sample/symbol =4, symbol_rate=1Mhz.

my gec filelink :

I run and get fft waveform bounce up and down, not stable.

here is  waveform record  link :

if I change sampling rate from 4Mhz to 1/2 Mhz , fft wavefrom become stable .

does anyone know the reason / solution ?

 looking forward to reply.thanks!