I had a similar problem but my speedo didnt work at all and the fuel gauge would go up and down when cornering. I replaced the speed sensor on the gearbox but nothing changed. I then started to check the wiring and found that it had corroded just underneath the battery tray where it joins the main wiring loom so i cut the bad stuff away and rejoined them with small crimp terminals and taped it all back up and everything works fine now. Hope this helps