Have you tried connecting the laptop directly to the projector with a known good VGA cable? I'm assuming that there is a 'plumbed-in' VGA cable from the projector down to the IWB.
Also, have you tried cycling through the display modes with the laptop connected? i.e. ltop + monitor ==> monitor only ==> ltop only ==> back to ltop + monitor. I have seen a similar problem with some recycled HP laptops and cycling through the display modes cures it.