I'm late to this thread (four month CHA message backlog, actually), but am interested in the topic. Did you get a new unit? Did that solve the problem?
I've had similar problems with Powermid-type IR transceivers. I found that the unit could NOT be placed on top of the TV, even in a completely dark room with the screen blanked, which caused me to conclude that the IR unit was reacting to EMF radiation from the TV. It also happened when placed on top of my receiver. Is there anything like a TV or an amplifier or receiver near the receiving sensor?
Since 2 of your units exhibit such behavior I'd look for something capable of emitting EMF near either unit or near the wire connecting them.
If all else fails, borrow a nightvision scope. If there are any other IR emitting objects in the room, they'll pick them out, at least mine does. The IR illuminator on my outdoor CCTV camera makes the outside look like full daylight using a nightscope. I've found some red indicator LED's on electrical equipment that output quite a bit of light in the IR range, at least that my nightvision scope can detect. Where are the flickering sensors located? Is it possible they are receiving some sort of IR?
I'm surprised ferrite beads didn't work. Where did they have you attach them?
It makes me wonder if your experience implies that for a foolproof IR setup you need to run shielded wires instead of ordinary CAT-5. As you point out, it's a little late now to know that you would need to do that if you wanted to be sure to eliminate this kind of annoying behavior.
Please keep us posted on the final resolution of this problem, if there is one!
-- Bobby G.