Thermostat high reading

What you're seeing is called heat soaking. Is your check engine light or service engine soon light on? If so and you're getting a camshaft position sensor or crankshaft position sensor code I'd suspect a worn timing chain. Especially if you have high mileage. This can also be caused by a cooling system that's blocked with rust and other deposits. Are you having any overheating problems when driving or idling? If it heats up when driving that points to poor coolant circulation. A blocked cooling system or bad water pump is suspect. If it heats up at idle it's an airflow problem. Faulty cooling fans, dirt and debris in front of the radiator, or a missing fan shroud is suspect in that situation. One other possibility is a knock sensor picking up the noise of a valve tap. When this happens the knock sensor will severely retard the ignition timing. This will cause heat soaking and loss of power. Hope that helps! Jim