It sounds like the server is disconnecting you due to inactivity.

Different versions of mIRC generate different amounts of traffic either by design or depending on your settings. For example, the latest version of mIRC sends ping checks only every five minutes. This could very well be too long for some servers. Some servers might allow five minutes of inactivity before disconnecting you, others might allow only two minutes. That's probably why some users never see this issue, while other users see it all the time - it depends on the server.

If you enable the "Check connection time out" option in the mIRC Options->Connect->Options dialog does that help? If it does not, other users find that they can resolve the issue by using a small script that sends a ping to the server every thirty seconds or so.

In the next version of mIRC, I will be increasing the "Check connection time out" rate to once every minute (it is only sent if there has been no client/server activity in that time).