From your description above, it's not the powerhead giving you problem. Its in fact the water level and evaporation problem.
When your tank water level reaches the top of the overflow box, it will overflow into it. When water evaporate, the water level in your tank remain unchange because the height of overflow compartment keeps the water level constant. Note: Evaporation will not cause your overflow box to become shorter in height. The the final water level change will only occur in your overflow box, and this explain why the ph is reaching the top of the water level during evening time.