I had an 89 FJ62 with 180K and had the same problem. Both the temperature and fuel gauge would spike usually when pulling up a hill after warming up. This would happen every time I went over Santiam Pass to Sisters. The first time I freaked out and pulled over to check and everything was fine. Out of caution I replaced the thermostat and still it would happen. I then found the threads here about a loose ground that was common to the fuel and temp gauges on the back of the cluster. A well placed hard thump on the top of the dash would usually make them back down. It would only do it once or twice and then be fine the rest of the trip.