PUN room disconnect player frequently after reconnect

https://forum.photonengine.com/discussion/14925/pun-server-disconnect-player-frequently#latest
Hi I have made a discussion of same question before and I still have no idea to solve it.

I am using PUN classic v1.98.
When a player disconnected once/reconnect to the server and room, the player sometimes get disconnected from the server by the cause "DisconnectByServerTimeout" frequently like 30 seconds once. (and trigger my callback to connect again)
Between two disconnections, I have not raised any event but the disconnect is still keep triggered.
If it is a problem of buffer full, how I clear the buffer to resolve it?