yah if you check out
http://www.jimwoldtechnologies.com they sell chips and computers and many other various things for the z31..
is this problem when you first start your car in the morning or the day or is it completely intermittent or all the time???
did you check out the speedometer cable to see if it was cracked or loose track it all out and follow it inspecting it very carefully because that is most likely your problem maybe youll get lucky and its just loose..