So, I've done some experimenting on my own and discovered some interesting stuff.
In July, I took a ride out to Colorado. Rode across Kansas in 100+ degree heat. While in Colorado, rode at elevation with temps below 60. On both occasions, I was able to get the speedo to stick and also to fix it.
Bear with me, because the rest of this sounds abso-freakin-nuts...
I bought myself $1.69 worth of white Scotch Adhesive Putty. Yeah, the stuff you put posters on the wall with. I pressed out a circle of putty about the diameter of a quarter. I then placed said putty circle over the speedo dial so that it created a shadow over the little chrome bling piece that caps the speedo spindle. It fixed the problem for me every time.
Here's why I think it worked:
I only really had the problem when the sun was high in the sky. In July, at my latitude, time of day was between 11:45am and 2:45pm. If the speedo's chrome dome bling part (yep, that's the technical term) was left in full sun, the speedo stuck.
I think, between the glass cover on the whole face and that little chrome dome piece, it was lensing the sun's rays onto the speedo spindle making it swell up and stick. As long as I kept the damn thing in the shadow of the putty circle, it quit sticking.
I must be on to something because Triumph asked to switch out my speedo cluster for a new one and wanted me to send mine to them with the putty in place. Even had a guy from Triumph call and talk to me about it.
But, I did get a new speedo with zero miles on it. Was like getting a new bike, right??