When I first got my truck the thermostat was bad, never heated up. After replacing the thermostat I lost a coolant line and heated the truck up. After that the temp would go from almost overheating, the L on normal, to cold again. This would keep repeating endlessly, ended up when overheating due to loss of coolant I almost toasted the thermostat.

 

After a very high heat event &/or multiple high heat events, Thermostat function abilities are greatly compromised. Replace it. Any question as to the proper function of the radiator cap (should be a 13psi), replace it.