Ok. So, the bleeder has to be through the radiator cap. It's been replaced, still happening. Flushed radiator, it's free and clear, still happening. Changed water pump, still happening.
Like I said, you're driving along. Comes up to operating temperature. Everything's fine... then, all of a sudden it starts to creep up... if you turn on the heat, it happens faster. If you push the accelerator, it cools it down???