Typically improper variation in idle speed under load is due to mixture governing problems. Too much of the mixture is being determined by the flyweight governor and not enough basic rack correction.
Not something you should screw with. Just as possible and maybe a bandaid even if not the problem, would be ignition timing. Give it 5-10 degress more timing and see how it goes. It should at least smooth it over till the tech can look at it.
Bosch Master, ASE Master, L1
33 years MB technician