97 frtl/ det. temp gauge goes from 170 to 250 plus or drops dwn to 100 but never overheats. whats wrong?

Have the gauge and sending unit checked. Detroits run hot, but should never get to 220. (Mine with a heavy load on summer time mountain grades has hit 217 when the outside air temp was 100+) The dropping down to 100 tells me the gauge circuit is shot. On a long downgrade in cold weather, mine never goes below 170. The thermostat in a Detroit normally opens at 205.