First response from the dealer yesterday was that the problem (according to Triumph USA in Georgia) is possibly being caused by loose connections/pins.
Fine... they can try that. I am not going to hold my breath on it though. I will keep the idea of the gear position and road speed sensors in mind, however.
My guess on that, if it were either of those sensors, is that it would be the road speed sensor, as it didn't seem to matter which gear I was in, or if the bike was even completely stopped and kill switch applied... the speedometer would be stuck at some value other than zero.
I agree - mine screws up in any gear when it takes the notion to get wacky or sticky which is currently often - It also screwed up when I got snowed on at MT Evans CO at 38degrees F so it isn't the heat, at least in my case. My dealer says he isn't getting any help with this and I reported my first experience with this three months ago. I currently have about 6K mi and now the speedo seldom works correctly.