Is the trunk loose ? The gearbox has a set of internal grub screws that often become loose and the shaft looses it's grip. There is also an opto interrupter that may need tightening. I can't recall if there is a "Trunk Test", but you could try entering the diagnostics and testing the trunk and optos.
Covered here on page 2
http://aussiepinball.com/index.php?topic=6943.15
