Spring tension isn't the only thing worth looking at. With the car only doing this at the top floor, I would think the weight of the ropes might be giving it a little extra. I know you said no indication that it's pulling through the brake, but intermittently is hard to tell. The gap between the brake coil plunger and the stop should be about a 1/4" or so. I always use a 7/16" -9/16" wrench (flat open end) to set the air gap. What can happen is the plunger isn't hitting the "sweet spot" of the coil and not getting enough pick. A lot of folks will solve this by moving the tap on the resistor, but those are solid brakes if mechanically set up right. How's the brass sleeve and whatnot? clean and free?

Also, sounds dumb, but check your spring tension under the car for the selector. Should be about 18-21" if you didnt know that already. This can cause a ton of weird issues that normally get adjusted around instead of fixed.

And.....It's always a good idea to clean and make sure contacts are hitting your floor bars like you were thinking too.