I was going to say it is to do with you disk as you and me both run rear disks, but then you said it has been like it for 4 years. When i first put my disk on, i had it set really sensitive, and it was rubbing slightly, as soon as i stop peadling, the wheel would stop spinning the the rear end would instanly drop down, as soon as i stopped the disk rubbing and made more movement in my lever it stopped doin it, and i could lift the bike up and gap properly. I think its to do with you needing to have the backwheel spinning in the air to gap or lift the rear end high. Not sure if that helps.