Sounds more like coolant level is low in the engine block. If you have a small leak, the cooling system will draw air through the leak instead of siphoning fluid back from the overflow tank.
Just checking the fluid level at the overflow tank is NOT good enough, you have to remove the pressure cap on the radiator and check inside the pressurized part of the cooling system, to make sure the coolant level is at its proper level. Make sure you only remove the pressure cap when the engine is cold, remove it while its hot and you'll likely burn yourself badly when scolding fluid burst out under pressure.
If Coolant level is too low, there often won't be enough fluid to circulate it through the heater core. And the engine temp will start to climb, BUT NOT overheat if the only problem is low coolant level.
If it was a thermostat problem, the coolant circuit for the heater core is on the engine side of the thermostat, regardless of what the thermostat does, the heater core should reflect the same temp as the engine. So if temps for the engine start to climb, then the temp in the heater core should climb with it.
Your seeing the opposite, as engine temp climbs, the heater core gets cooler, sound more like low fluid level, and you notice the conditions where water pump isn't spinning fast enough to circulate the low fluid level completely through the engine, it just splashes it around.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.