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Interesting situation. The Prius uses throttle-by-wire, meaning there is no throttle cable. Many other vehicles operate the same way. Assuming all you have reported is accurate, there appears to be two possibilities:

1) There is a problem with the electronic throttle actuator, not allowing it to be properly controlled by the cruise control electronics, which I believe are built into the powertrain control module, or a problem with the electronics themselves.

2) There is a problem with one of the vehicle speed sensors, or the computer which calculates it, resulting in an inaccurate speed being fed to the powertrain conrol electronics.

If you are having no problems controlling your vehicle speed when cruise control is off, you should be able to safely drive the vehicle. However, the "wait and see" approach of the dealer is, in my opinion, unacceptable. They are paid to diagnose and repair problems. I would tell them to get busy and do their job.
 

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The stalk control suggestion is a really good one. If you experience the problem again, try an experiment if you have some distance in front of you. As I understand it, you are experiencing a run-away situation, as if the cruise is stuck in 'accel." If this happens again, rather than cancel the cruise or powering it off, try the tap down feature. This will allow for that contact to be made and should result in a return to the original set speed minus 1 MPH. If that happens, you have confirmed it is the stalk control. If nothing changes, we have some more pondering to do.

On the MPG screen, you should see MPG of 100 during the downhill portion. Monitor that screen when you try cruise again. If you are in accel mode, MPG should be something less than 25, confirming high throttle position.
 
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