Contacted AppleCare this afternoon and the technician asked me immediately to perform Speed Tests on both the upstairs iMac and the HT's Mac mini. Turns out that the downstair's was using the amazingly slower of the two. Using the Network, which had to be set up by another Apple Tech when I first installed the Airport Extreme 802.11/ac, was producing speeds in excess of 17Mbps, which incidentally took him ages to complete. After the downstair's ATV was added to that same Network, its signal strength also climbed to >17Mbps and all the problems associated with the ATV3 were resolved. I'm still at a loss to reason why the ATV3 began to perform as it did in March for no actual changes were ever made to its setup to bring about its errant behaviour. Thanks to all who took the time to respond to my problem.
Wireless is subject to all kinds of problems. I figured out that my wireless suffered when I turned on the lights. Turns out the LED EcoSmart bulbs at home depot cause craploads of intererence. I had 15 of these bulbs! Once I discovered it I unscrewed all the bulbs and my signal improved drastically. If I would screw in any 1 of the 15 bulbs my wifi sucked and my TV antenna would also go out.
Point being, there are lots of things that can interfere with wifi. If you are noticing intermittent signal drops, try making sure absolutely everything else in the house is off, including lights, then go around turning things on 1 by 1 until you see a signal drop.