The motor will rise in temperature after turning off, because the water pump stops moving the coolant.

A long shot, but, you aren't topping the radiator to full everytime this happens are you? You are not meant to fill a radiator to the top, but leave a few inches for expansion.

(In laymans terms)

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306D 1975 (camper) - rusting away
W116 450 SEL 1975 - sold
W114 long wheelbase ambulance, 3 litre diesel 1974

VW Golf 1 convertable - midlife crisis item
VW T4 van - support vehicle