Outer Limits 4x4 Board

Hi to all
I have a 1990 gq lwb, the problem is that the trip meter and the odometer decide to work only when they want to. The needle always indicates the speed correctly its just the km's that done roll around all the time.
There doesn't seem to be any reason to why it stops or just starts again. sometimes its while driving other times its the next time i go out.
Help would be good as i like to work out my economy each tank and fool myself that the 22l/100 is great as the tank before was 22.3l/100.
Thanks for any ideas

the electric motor that runs the trip an odometer isprobably on the way out or theres a bad connection .
If you can get hold of another motor from a wreck its about an hours work to replace your old one.

Thanks for that i will take out and have look