I'm back to retract my previous claim that my sensor problems were the fault of knock-off Unos. I was wrong. And to Nick_Pyner-- obviously open source hardware can be copied and sold. But I don't think it's inappropriate to refer to those other than genuine Unos as knock-offs. I doubt if anyone would be confused with what I meant. I didn't state the supplier of my collection of knock-offs (KOs) because I've accumulated them over a few years from here, there, and yonder.
But my point tonight is to retract any hope for solving sensor problems (as described in the OP) by ordering the Genuine Article (GA). The KOs were all wired up to a board I made with pots and LEDs, etc. When I tested the GA, it was hooked to nothing but the sensor and PC. I repeated the test tonight with a KO disconnected from my contraption and got good numbers. So the culprit must certainly be in my homemade test board.