The vehicle is a 1998 Pontiac Siluette Van. Overheating and engine is shutting down. There is no evidence of coolant leaks in the engine compartment, under carriage. There is air bubbles coming out from the hose that is connected to the reservoir. I suspect the thermostat being at fault, looks like there is no coolant circulation between the radiator, the engine ane the heater inside the van. All hoses, belts and water pump are ok. Any suggestions?