What is causing the engine temp gauge to skyrocket then cut the engine only when your revs are low and the heater to stop working as the temp gauge starts to climb on your 1999 Pontiac Montana?

The sudden rise in the engine temperature gauge and subsequent engine cut-off at low revs in your 1999 Pontiac Montana could be due to a failing thermostat that is stuck closed, preventing coolant circulation. This can cause overheating, especially at low RPMs where the water pump may not circulate coolant effectively. Additionally, the heater stopping might indicate a lack of hot coolant reaching the heater core, which would occur if the engine is overheating. Other potential issues could include a malfunctioning water pump or a blockage in the cooling system.