Check your grounds going to the cluster, you may have an issue there. Also, if there is a 'voltage stabilizer' wired in, see what voltage it's giving the gauges to work with. Some clusters don't get 12-14 volts from the altn, but instead, have a steady, lower, 'stabilized' voltage sent to them to prevent the problems you're seeing[erratic operation] I don't have any books for the earlier Gen 1 Monteros-so I can't help much

'92 SR Montero, Green, with 31" Yokohama Geolandar A/T Plus Tires, Mitsu factory fog light kit, on-board air compressor,Sony Radio and 10 disc changer. 213K and counting.....
Stock & Happy otherwise