- you can put a shielded ribbon cable in place for the one that doesn't self-contaminate. Ground the shield at the signal source end, usually "in the field".
- the cable with the self-contaminating 24VAC needs work. As a first attempt, you might simply separate the wire pair with that signal, and try shielding the rest of the cable, leaving both connection ends as is. This would involve wrapping the remaining wires with foil, and adding a ground connection to the foil at one end. But it's really not ideal, it would be stopgap.
The better solution is to remove that pair from the equation. snip the offending wires at the source end(being careful to only cut the two). Now, shield that cable.
Run the 24V signal as a separate cable, via a separate route.
To this, you might be able to add RC filtering at one end or the other, by creating a small plugin board and filtering the signals appropriately there. Total cost unlikely to exceed $100, if you know what you're doing.
Good luck. These problems are vexing.