I'm noticing this really strange gremlin after I've parked my car on a declining stretch of road (nose down):
upon starting the engine, the 'low coolant level light' stays on, and the engine idles very rough, as if running on three cylinders (out of four). After 20 seconds the idle runs fine again, and the coolant light goes out after driving some 50 yards. Coolant level is fine. Sometimes the rough idle also happens on starting without the coolant light on.

What could be wrong here??

__________________
2011 Prius