If you are saying that there were thousands of lines in a window, and when you returned to it later there were only ten or so lines, then there is clearly something wrong. This does not sound like the same issue that was reported at the start of this thread however.
Since you are using mIRC 6.2x, I would recommend moving to 6.31 which has had a whole range of fixes and changes which might have already fixed the particular issue you are seeing. If you are still able to reproduce this in a clean install of mIRC 6.31, it would really help if you could describe a step by step method that reproduces this issue for you.
