This is what makes me think it's some kind of loose wire behind the empeg.

That is a possibility. There were some of the sleds that had bad crimps on the connector and the wires would tend to pull out of the connector block. I'd definitely check that first, it could account for everything.

Can anyone give me a firm answer on whether or not I should ground the empeg sled to the chassis?

There's no firm answer on this as it'll vary from car to car as to whether or not it's needed.

In theory, it should only need the ground from the sled unless there's something else in the system causing a ground loop. Sometimes ground loops can be solved by grounding to the chassis, sometimes they are solved by isolating the ground from the chassis. You never know, really, until you get into it.

But before worrying about that, I'd triple-check all the wiring, because the way you describe the problem being intermittent really does make it sound like a simple loose wire.