Sounds like a classic rediator problem. As with most things, the Mercedes seem to have 8 million different ways something can go wrong. But this sounds like a classic radiator problem. The rediator is clean enough and efficient enough to cool the car at low load but as soon as you call for more power and heat the radiator can not disapate the heat fast enough and you overheat. Had this exact thing happen with my Nissan (89), could idle all day in 100 degree heat with AC blasting, get on highway and instant over heat.
No one likes to replace a radiator (cost) but sounds like you might need to.