I've never worked on the ZF automatic, but most automatics that I have dealt with that aren't computer controlled all use some kind of vacuum solenoid to vary the shift pattern.
But I'm more thinking that the vacuum situation is causing the issue. Not only do you increase the vacuum in a long downhill (vacuum does go up on a downhill, right?), but you're also at low throttle, so you're probably over-fueling the engine. The fuel pressure regulator is also vacuum controlled. Doesn't the pressure RISE with a high vacuum? This would further increase the over-fueling issue.
Just guessing, but that's my thought.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.