AHA. Actually, I'm convinced that I just nailed it. Nothing like a holiday at home to give you time to really mess with things. There's a wire that runs from the RFX-150 to one of the MRF-250's. It supplies power to the RFX-150, and carries the data signal from the RFX-150 to the MRF-250's. The RF light was glowing on the RFX-150, indicating "interference," and I put my fist tightly around this wire; the RF light got a lot less bright. Clearly, the interference was getting to the RFX-150 through this wire, not through the air, and my fist was acting as a choke. I tried turning components off one at a time, as suggested in this thread. Turned my receiver/power-amp off, and the RF light on the RFX-150 went totally dark. Turned it back on, interference back. Off again, interference gone. Repeatable experiment! Clearly, when the receiver is on, interference is inducting its way into that wire. Why? Because the damn installer had bound that wire into a tight bundle with a million other wires, most of them coming from the receiver. Goddammit. I undid all his plastic straps and freed the RFX-150's wire from the bundle, and gave it a nice route to the RFX-150 all by itself, and suddenly the Replay is more responsive than it's ever been. I guess it's just more sensitive to "imperfect" signals than some of the other equipment.

The lesson for me is to make sure I've employed a real pro next time...there's more to this than just hooking the right cables up to the right jacks.

And for you guys, if you have any doubts that induction is real, and can make or break a setup (and not just cause a little "hum," this, as far as I'm concerned, is proof.

(Of course, if it turns out my diagnosis is wrong, and the intermittent problem comes back, I'll post again to let you know...but I think I've got it this time.)