ERROR: Your host is trying to (re)connect too fast -- throttled....
An IRC server can only accept so many connections at a time from one host or IP address. This is a security measure against the various forms of clone bots and flood bot abuse of IRC, and is known as "throttling."
Problems with getting throttled accidentally may be caused by the fact that your client retries to connect too fast, Set up your IRC client to wait a period of 20 seconds before reconnecting.
If you're using mIRC 5.91+, you're in luck. As of version 5.91, mIRC now waits 30 seconds before a re-connect attempt if the server says that you were throttled for connecting too fast.
mIRC now looks for the word "throttled" in the ERROR Closing Link server message.
* Specific to Undernet:
Connecting 3 times in a row to an Undernet server within 20 seconds will get your IP throttled for 20 seconds.
You should configure your IRC client to try connecting to the next server in the group (ie Dalnet, Undermet, Efnet) to make sure that eventually your client will reconnect.
I hope this helps get rid of your annoyance.