PTP time sync in slave device not working?

I am trying to sync hardware clock with software clock in slave device in ptp setup, but the offset value suddenly jumps from 28 ns to -617 ns. I tried varying pi_proportional_const(0.7) and pi_integral_const(0.3) to stable the variations, but it didn't work. * sync command

-sh
    phc2sys -s eth0 -w CLOCK_REALTIME -O 0 -m

* log details of sync command

phc2sys[1289.198]: CLOCK_REALTIME phc offset 28 s2 freq +976 delay 6500
    phc2sys[1290.198]: CLOCK_REALTIME phc offset -617 s2 freq +339 delay 3541