When you pc someone and have a reasonable long conversation with this person, if you scroll up the conversation like you were looking for something and hold the scroll bar, the chat box will give those "pulses" to make you go to the end of the conversation. These pulses are 5 seconds apart.
Alternatively, I suggest removing these pulses completely, allowing the user to completely browse the history without any problems. There could be an arrow icon, which will appear when there are new messages in that private chat. Even with this arrow, the chat box would not force you to see the end of the conversation.