1198 si gps speed

I have a 998si it has gps right with my buddies I-pilot and the speed does not jump around it is steady with trolling . It adjust real slow but the 1198si I just bought jumps around all time and is off by .6 mph compared to my other unit. Is there any way to adjusts the speed. It will be .2 on 1198 and the 998 will be .8 . I longlining most all the time would like to have the speed on 1198 better. I still use a hand box control for more speed control. I not a fan of I-pilot in wind better control with hand control.Hope someone has some ideals to help me out.

You ever run a drift sock to hold boat more stable when trolling?

What software version is in the 1198 ...??

There were some software issues in the GPS speed in the late xx7 series and early xx8 series units ...

http://www.crappie.com/crappie/fishi...tymie-waiting/

Rickie