I have tried to search for my specific issue, but was unable to locate anything.

I have a 2001 Cherokee Sport, AT, 2wd...

I have changed the Radiator cap, T-stat, water pump, and had the system flushed. The sending unit appears to be new-ish (its all shiny with the original orange sealant on the threads). The coolant system has been checked for exhaust gasses (none detected).

When driving down the freeway, or in the city, the gauge quickly reaches 210, then slowly climbs to the next tick-mark (~222?). it gets maybe a needle width past that mark, then jumps to the first red mark (not pegged at 260) and the 'check gauges' light comes on. If i turn off the AC, it will drop back down to the 222 mark and just hang there. The guage NEVER stops anywhere between the 222 mark and the first red mark, and never goes over the first red mark.

Last night it did the same thing driving home without the AC on and I had to turn the heater on to get it to drop. The heat blows 'warm' but not hot. The lower heater hose does not get hot.

I believe the radiator and heater core are clogged, causing it to run over 210, even at night with the AC off- but what would cause the gauge to jump like that?