Ron,
You are right about the upscaling/downscaling. In our case, our sensors downscale. I'll be more specific about the setup - there's a sensor, connected to a transmitter. The transmitter has a scaling device connected to it. The scaling device takes the raw mA from the transmitter and converts it to a 4-20mA signal. This 4-20mA signal is what goes to those sensor satellites I mentioned earlier. When the processor of this scaling device sees that the scaled mA is below 4.0 (specifically it's 3.2 mA) it goes into fail. For a while we thought that there was an issue with the scaling, but we know that it's doing what it is supposed to. When we take the mA reading at the transmitter, we know that there has been a drop. This is basically what has us stumped - what is causing mA to drop for random sensors. This would be much easier to troubleshoot if all sensors behaved the same way.
So yes, you're absolutely right, something strange is going on here. We have these sensors in hundres of different locations all throughout the country. So if there was a problem with components, we would've seen this somewhere else too. Also, these sensors have been made the same way for over 20 years! And this is the first time we've seen something like this. We keep thinking that it must be something to do with the wiring at the plant, but just can't quite figure out what exactly it is.