I don't know alot about autos, just the basics, so please correct me if im mistaken.
But he says it is problematic on the way up to 80km/h? once it reaches cruise speed, then it stops? Is it possible that once the torque converter locks, and he is just cruising, then the problem is gone?
Again, im only guessing, because i doubt its coils or spark plugs. When coils died on my car, or my brothers car (he has an 05 vz wagon, still the same motor as mine or as the OP's) there was no gradual decline, they just failed there and then within about 10 mins of the first miss. And more importantly, the engine threw a code straight away, and ran like a dog, whether idling or driving.
It could also be the PCM, if it's failing to send a signal to any of the coils, but again the knock sensors should pick this up and throw a code...
Again, I'm not an expert, but i'd get the auto checked over