I've encountered the same issue on my 156 SW 2.0 JTS '04. When driving on cruise control, around 80 km/hr, and when going slightly uphill, the car first starts to accelerate. This is for only one second, and then suddenly all power drops, the engine light turns on, the dashboards starts a peep sound. Basically the engine shuts off, because all power is gone, engine does not react to gaspedal or anything. Had to pull over, turn the ignition key off. After one or two tries, or after 10 seconds or something like that, engine starts and you can drive off again, (watch for upcomming traffic!!!). Engine light is still on and goes away after couple of times, parking and start again.
I've ONLY encoutered this while using cruise control. I've driven many time on the same road without cruise control without the issue occuring.
Engine fault code with OBD is P1337. Haven't found a solution, only the containment: don't drive on cruise control when going uphill.