I recently replaced my radiator, flushed everything 3 times, and burped the system but when I drive, the temp needle will be in the 11 oclock position but when I start to push it, the needle will gradually move to the 1-2 oclock position until I turn on my heater.

What would be causing this? did I not burp the system long enough? I did it for 15 minutes, the coolant in my funnel did not budge at all, no bubbles.