Do you actually disconnect from the Internet? The lag bar will increase if you have Remotes turned off due to the fact it's not checking your ping reply and so it just assumes you're not replying. Make sure Remotes are on.
On the other hand, try /remote off and see if it is a script causing the issue. Try installing a fresh mIRC. If you've installed other scripts with PnP you might try unloading them as PnP is known not to be too compatible with 3rd party addons.
Does this happen with every network you try? Do you disconnect from any exterior programs such as MSN Messenger, AIM, ICQ etc.?
Good to see you posting again btw