When was the WP last replaced?

A complete shot in the dark...but slow flow, or less volume, through the radiator would allow the coolant to cool off a lot more during the time it's in the radiator. Which will give you a greater delta T. I suppose an obstruction in the system could do the same. Or a barely cracked t-stat, but you've replaced that a couple times.

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'08 X5 4.8i - 184k
'15 i3 - 92k

'98 323is - 169k - Garaged
'94 325i - 208k - Sold