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To me this sounds like some kind of a sensor issue. Almost sounds like PROCHOT is being asserted. There are several resistors around the SMC that have thermal sensor signals going through them. You need to check them. PROCHOT is asserted by Q5159, U7200 and the SMC. Have a look at those components and see how they look.. You need to look at all the resistors around the SMC and measure them. It will be time consuming but the issue may very well lie in one of them. Since the CPU would have vcore if PROCHOT you would still get a light but it would not get hot as it would be throttling vcore heavily.