Door lock contacts kits are cheap once you start dealing with a persistent callbacks. I'd replace them all on every floor to be sure. Or at least clean them all with emery cloth followed by contact cleaner.
Had an Ibox once that had a bad spot in the fire wire for the bottom floor lock that the car would randomly stop around 3 or 4 when that wire lost continuity, 8 stop car. Had to pull new fire wire from machine room to basement. Just brainstorming.
I agree with Montana, check everything. Pull duct covers off, see if they had lock wires connected (wire nuts/steakons) in the duct that could be checked. You can't rule out anything If you didn't install it.
I also would ride that car on inspection and as you go violently shake each door as you go by it, looking for one that stops the car.
