Thanks for all your help. Here's a quick update:

I just went out to examine the riocar and sled again. The sled _does_ have a lip on all four sides.

Moreover, I noticed that it only takes a very light touch to interrupt the power. There's maybe a 3 mm gap between the exterior lip/edge of the sled and the mounting dock of the car (which is made of metal, btw). When I push on the sled/riocar and the sled's lip/edge come into contact with the mounting dock, the power goes out. This little push is too insignificant to loosen any connections in the sled's main connector, IMO. It is seriously a very short distance, like I said maybe 3 mm and doesn't seem like enough of a movement to cause any disconnections.

Does the sled require a ground connection? I don't have one, just on my receiver. Could that be it?