When you're not actively chatting (Idle), the server you're connected to pings you every few minutes (3 minutes I believe) to make sure you're still there and didn't get disconnected. When mIRC receives this ping (PING?) from the server, it responds with a ping reply (PONG!). If the server doesn't receive a ping reply in a set amount of time (3-4 minutes I believe), it closes the connection with the "Ping timeout" message. Server lag sometimes causes these transactions to be delayed which sometimes results in a ping timeout. Try connecting to a different server on the network, or even connect using a different port on the server other than the default 6667. Most servers listen for connections on 6666,6667,6668,6669 & 7000.
~ Edit ~
mIRC actually has an option to check for a timed out connection, whcih may help you with your problem. Options (alt+o) > Connect > Options > select 'Check for timed out connection'