ok ill tell ya straight up it may not me mirc entire fault but ill see if any one else can reproduce it. in running windowblinds which may be a factor but i doubt it very much.
leave mirc connected to a sever in a midsized (3x people) half busy channel for 22 hours and try to scrolll up with a wheel mouse or the scroll buttons, or drag the scroll slider. it tries to resist scrolling. mirc has the default buffer line settings also