Thought I'd have a look through some old posts.
I had this exact problem yesterday. Except it would jump out of gear every time after I was sitting idling at the lights, or wherever, for a few seconds. I tracked down a leak I had from a small seal in the bottom of the auto tranny and most of my fluid had leaked out on top of the exhaust, which would have looked pretty cool for people driving behind me. It's darn awkward to replace the little thing, but I got quoted $15 so it's not even worth buggerising around underneath the car when I don't have a hoist in my back yard. We just tightened up everyting else while we were there though.
Alas, I will miss those moments at 40km/h when it shifted into neutral, revved it's tits off, and then shifted back into gear again and laid a bit of rubber. That must have looked impressive from the side of the road in a standard EB.