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My vote is that you have a very small vacuum leak in the intake tract after the TB. Check the hose connecting to the air oil separator for cracks or a bad seal. Next, check the rubber boots connecting the intake plenum. Any of those will cause the engine to still be getting air, making the revs not drop when you let off the gas. With a really small leak, as the engine heats up, things will expand, closing the leak.
Happy hunting
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