80 series landcruiser speedo problems

Hi, can anyone "help" with a problem I am having wit the speedo on my 80 series. I have a 1Hz turbo diesel and the speedo works sometimes, like it will work for a couple of weeks and then just not work for weeks...occasionally it will jump or flicked for a second during the non working stage. Its the electrical sensor type and there is no speedo cable at all.

Anyone know what to look for?