I can nudge my cruise control speed lever and my speed barely goes up, say from 80 to 81.I nudge at again and again, up to 83. Then I nudge it again and the car takes off, no speed limit. Nudging the cruise speed control lever down has no effect until I've done it about 10 times or more. By then my Prius is doing 97. It's scary because it's so wrong and so out of your normal control
Having an older Toyota truck and being someplace you can safely drive 85... I'd have to say this sounds like a misunderstanding of the cruise control. I have experienced the very same thing, and it can be scary, but it is NOT an ECU problem (at least mechanically, it could be argued to be an interface problem). It is an impatient driver problem. Using the same scenario you just describe imagine the following:
You are driving up a long gentle grade on cruise control at 80MPH, perhaps with a headwind. Unbeknownst to you, your throttle is already wide open as the cruise control maintains your set speed. You bump the cruise control up two notches, but you do not appear to speed up. You bump it up two or three times again and your car barely accelerates to 81... You bump it up another 5 times and creep up to 83. Now you are really impatient and hit it another 10 times with no apparent affect.
Sound familiar? Now think about what you have done, you have set the cruise control from 80 to 100, but your engine is maxed out maintaining 80-ish. As you start to crest the grade your engine has more available power and the vehicle takes off to reach the new speed you set the cruise control at, 100. As you accelerate thru 85, then 87, then 89, you bump the cruise control down and then again and again, but your set at 97 now so you continue to accelerate.
This is not a problem of the ECU "going crazy" and taking off... this is the driver losing track of what they have told the ECU to maintain. The solution is simple, pull down on the cruise control lever as many times as you pushed up. Or. more preferably, pull back on the cruise control lever to immediately cancel the set speed. Or, step on the clutch to immediately cancel the set speed. Or, step on the brake to immediately cancel the set speed.
Having a 10 year old 2WD Tacoma truck with a vacuum-actuated cruise control throttle, this can be even more fun. With the throttle wide open the actuator is at full vacuum and can take a couple seconds to release when cruise control is canceled. Having new Prius with "drive-by-wire", the cancel effect should be almost instantaneous.
Neither my Toyota truck nor the Prius are speed demons. Yes you can get them both up past 100MPH (OK.. so Ive only had mine to 97), but neither has the power to maintain that speed up grade. You can always ask the vehicle to do more than possible at the moment, it will happily oblige when the power later becomes available.