Connections during speed change

[talentplace]

I have a same situation, but it not depends on speed increasing. Part of the cars moves into one spline and another part into another spline. It's very strange becouse i've got this problem with one arrow only (i have very much arrows - about 30 and they work fine). I solved this problem in the editor by changing all the cars train UpdateIn method. The problem appear when it set in "Fixed Update" but when "Update" all work fine. But not in the build. How it can be that in the editor it works but in the build there is problem?
Also i think that this problem can be somehow related with the very big length of the spline, becouse this arrow is the most distanced. My spline's lengh is 18300 and this arrow on 17700.
Any ideas?
Thank you in advance.