Another long shot ... clogged overflow tube? Does the position of the overflow fitting on the radiator require a specific cap to intersect the overflow tube fitting properly?
If the cap seal doesn't lift enough to uncover the overflow opening it might retain pressure.
Just mentioning it because the wild card here is the radiator. The design of the filler neck is pretty important to make the cap work properly. The spring in the cap has to be compressed the right amount etc.
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