I added two pressure dampers to the sensing hoses. These are often used on the emissions plumbing of older cars.
If selecting them at a car dismantlers, be careful that they're not one-way valves (also often used on the plumbing). Check them by sucking on them (and if you inhale fuel vapor - that's your issue). The dampers will allow air to be
just sucked through them in both orientations. Conversely, the one-way valves will hold a vacuum when you stick your tongue over one port after inhaling.
The use of the dampers dropped the stagnation pressure variation (the largest value, and so the greatest variation of the pressures I measured yesterday) to about 0.03 inches of water, making it much easier to get a reading.