Mizchief; the esc won't hold power on so my bet is that your receiver "fail safe" setting is throttle input. This is often set when the receiver is bound to the transmitter. A quick look at the Rx8's LED tree would have likely shown a single led or even two when powered on indicating the signal from the receiver was indeed giving the esc the go ahead to give power.

Some radios after a few years even get some "wander" in the pontentiometer pot which can lead to this condition as well. Top end radios will have a "monitor" screen where you can see the raw data/voltage bounce around.