If mouse jumps all over the screen, or it is locked in the corner, or does not allow to have a good control, it is caused probably by autolock function. So try setting it to the opposite of whatever it currently is.
In your dosbox.conf file, look for a line setting autolock=true and change the true to false, and vice versa. Or, if no line with autolock exists, add one with autolock=false. It should help.