Here's a couple of things you could try:
1) all of our transducers are grounded at the panel only. I don't know what the difference would be between the transducer end or the panel end but the panel does have a dedicated ground for this purpose(green/yellow striped)
2) 300' to 500' is a long run. Don't assume the wire or connections were properly installed. Lift all connections and ensure proper torque when you retighten. I have seen entire installations have poor connections. This is a long run and will need every little free electron it can find.
3) do you have a handheld calibrator. You could try comparing machines side by side. Also comparing before and after the wire.
I'm sure after months that every little nook and crany has been checked. But when I get stuck on a hard one I try to go back to my basics:
1)Don't completely trust the operator or any technician before you. Take in all info but go with your best judgement regardless of how simple it may seem.
2)Most hard problems I solve are found by a combination of good troubleshooting and ROOTING around. Take the time to open all connections, don't just tighten, remove connections.
3)This is sometimes hard to do because of lost production but try to compare two machines piece by piece. Start at the transducer end and start comparing every little thing you find.
I hope this helps you out but I'm sure you've already tried most of this stuff. What immediately raises my eyebrow is the length of run. 24VDC can easily be affected by that length if there is a bad wire or bad connection.
Keep us informed, I'm curious.
