Boy it sure is hot
I had a 89 56-SEL that was running hot just as you described. Cooled great in town with A/C on (aux fans running, low heat load to dissipate), heated up on freeway in hot weather, then shot up on slowing down. Pulled the radiator and had it rodded out (shop that could remove and replace tanks) and solved the problem completely it ran at 85C.
Cause? Antifreeze additives salting out of solution from previous owner running it too long.
Your case --- incompatible antifreeze read the fine print. If you are using the 'green' stuff or MBZ stuff, you MUST completely flush it all out before switching to 'the orange' stuff. The two are incompatible and will plug up the radiator if even slight cross contamination. Cleaning agents will not remove it. Only recourse is mechanical rodding or replace the radiator.