Use a [set $1( message before the slider when updating from the player or other objects in your patch.
I tried this, and somehow it prevents the slider from updating 'itself' through a feedback loop ( It is getting the data from the player (veejay), and sending out data to veejay to change the position when I drag the slider...). Althoug it doesn't drag very smooth yet, it seems to behave more like a gtk slider already( that is, don't update from current frame until dragging stops)
But I don't understand why?