My S8 does this as well, but only in warm weather when stationary in traffic.
The thing that cools the temp down when it hits 100 or 110C is your AUX exectric fan coming on, check it out next time it happens.
I suspect the temp creeps up because the main viscous clutch fan has failed.
I bet if we all replaced this it would be back to normal.
Actually I am planning to replace mine with a main electric fan that comes on a bit lower than the AUX fan, which seems to wait until its over 100C
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