Resurrecting this thread, because it seems like the most comprehensive that my search turned up. Also, if anyone has found a solution please let me know.
My 2013 purchased last September exhibits all of this behavior. I've had a variety of cars and not one of them (even a rental hyundai) had such poor control algorithms. It is very disconcerting when the truck down shifts, and accelerates. Sure it may top out at 5 over the setting, but it doesn't feel like it, so I end up disengaging the cruise so that I am in control.
Not using cruise control is unacceptable. I have a 12 mile commute with about 9 miles on a straight shot with long gradual hills at 65 mph. I prefer to move to the right, set the cruise, and not fool with anything.
I'm going to try to do something about it. I've opened a complaint ticket regarding this (#14012190847) with Toyota customer experience. They referred me in for a service visit to the local dealer. I urge anyone that is dissatisfied with the cruise control to do the same. Might be an exercise in frustration, but the only way to get them to fix it is to document the problem. I will update when the dealer inspects.