P0492 parameter is basically for max speed difference detected by drive in per sampling time,This parameter sets automatically in the ID run of drive.As per the application you can change this parameter upto certain limit.F31118 fault is when speed difference goes out of the range P0492 then F31118 fault generated.
Kindly let me know the below details.
1.Which type of Encoder cable you used for encoder?
2.Is the cable laid in parallel to the power cable?
If the answer for above question is YES then do below suggestion.
1.Use Individual & Overall shielded cable,If you are using 3 pair cable & only 2 pair is used then 1 pair connect earth with shield & both side shield connect to earth.
2.Seperate the Encoder & power cable in different trays as well as at the point of crossing ,make it perpendicular.
I hope this will meet your requirement.
Finally run the drive in V/F control mode & check the encoder signal on trace.r61(0),r 60.
Check wheather any drop in encoder signal.