I haven't been back to the job, the shop has been slow ordering parts. Will post any findings when I return.

I kept referring to the board as an "MCC-GDP", it is actually called an "MCC-CGP" board. Either way, I changed the board yesterday and it didn't fix my issue. Controller power supplies measure in phase, I am out of ideas.

What is the MCC_CGP pcb? Is this the main pcb for the SLC(single lift computer). From what you have said, you have isolated the calls from the from the shaft so to me it can only be the SLC that is generating these calls. Have you disconnected the complete chain that goes down to the car calls? If not I would completely disconnect any and all down chain equipment (for the calls) so as really know where this is coming from, and see if these calls come in again. Then you can go back to the manufacturer with more info on the how it all works software/hardware etc.

I think its fixed.

I changed all ribbon cables with no effect on my problem, so I called MCE again. The troubleshooter had me check for a loss of communication on the swing panel of the group controller, turns out I was in fact losing communication.

MCE has two different types of "receiver-driver" chips on their communication (RS?) boards. Looks like the problem car and the dispatcher have the weak version of this chip and my nice elevator has the good version of this chip.

To eliminate communication loss, there are two jumpers on each communication board; they take a resistor to ground at the end of the communication link. I moved the jumpers on both cars, my communication is now good and my problem disappeared.

So, I ordered new chips for both cars (a few bucks a piece).