This is more believable to me, as the "set-point" for cruise control is not dependent upon throttle position, but instead, is remembered by the computer. There's nothing to check against to insure believability.

More suspect, in my opinion, is that in a cruise control, you have a feedback system, based on car speed. The car speed is almost certainly redundantly sensed, so that's no worries, but the feedback loop itself could potentially go oscillatory if there were other variables introduced that hadn't been designed for. Those variables could be pretty subtle. For example, maybe the gear motor that you use to mechanically control the engine gets sourced from another distributor, and they give you a better one, that has more torque. Perhaps that throws off the stability analysis that you had done. Tons of things could change somewhere between the 500K's car and 1M's car you produced, lots of different vendors and permutations could come into play that could throw off the stability of a cruise control, I would think.

EDIT: They're probably using feedback in the control motors as well (servos), so that's a non-realistic example, but it illustrates the problem.