Low idle vacuum:
- Major vacuum leak
- Late ignition timing
- Wrong static (cam) timing
- Low compression
- Mixture way off
Of course low compression is constant, ignition timing is non adjustable (unless it's always late because of a bad knock sensor), and static timing does not normally bounce around unless there is play somewhere like the cam sprocket bolts being loose.
Other things like collapsed lifters and sticking valves cause lots of vacuum bounce because it's only one cylinder doing it.
A large vacuum leak does not go well with smelling gas, nor does a leaky induction system causing a lean condition.
Keep thinking, and testing.
