Some display interference between the Canon UI and the CHDK UI is normal and unfortunately unavoidable. This because the Canon UI does not know the CHDK UI is writing to the LCD. There might be a software way to overcome that but nobody has figured it out so far.
The amount of interference varies between different camera models and is always worse in shooting mode than in playback mode.
If you have things like focus tracking or face tracking enabled in the Canon menu then it will tend to make things even worse - you might want to check for that on your A2500.