I left the gauge attached and took her for a drive, and its completely different results.
Upon acceleration it drops to 0 and stays there until I almost completely back off the gas. I then, for comparison (since my wagon has had cats replaced) plugged the vac gauge into my 300TE and did the same journey and she did NOT go to 0, but stayed around 2 then began to gain vacuum again unlike the 260E.
In addition to the vacuum gain, the gauge responded much quicker on the 300TE too.
Video of 260E:
http://www.youtube.com/watch?v=euhKbv7ECmA
Anybody think this points to blocked cats too?