Buco,
It sounds like your Z4000 is behaving as it should. As the voltage is turned down, it should suddenly drop to zero as you pass the 6 - 7 VAC range. My Z4000 behaves like this as does my old Lionel ZW. The PS1 can be a persnickety engine. Make sure you have a BCR installed to replace the battery for best performance.
If you absolutely need a proportional control that won't jump down and will slowly drop voltage as you turn the handle, the work-around solution is rather easy. Just output a fixed voltage from the Z4000 into an MTH Z Controller box costing about $85 USD.
(Picture from PatsTrains.com)
To be more technically accurate to your initial question, and if you were inclined to build something, the component you needed is called a thyristor rather than a resistor. (There are also other semiconductors that will work.) A thyristor would control AC voltage without wasting energy as heat. In fact, all my sidings are controlled by a type of thyristor. The Z Controller above behaves as an off-the-shelf and more expensive version of a homebrewed semiconductor circuit.
Hope this helps.
