Not that this is your problem but I had overheating on grades with a Poorly adjusted transmission. I recall the car didn’t gear down early enough on grades. It got bad in hot weather on steep climbs. Oh yeah another time I had a slightly loose upper hose due to a cracked neck on the rad that leaked when I climbed fast. Overheating went away when I fixed that.

There’s always the possibility of slipping plates in the tranny too.

Good luck finding the cause.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles