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You have an electrical problem in the car.
Sorry to be so generic, but there is no way to narrow down the problem without a thorough diagnosis.
With that said, it's a PITA to troubleshoot these kinda problems.
The tach dropping is an effect of the problem - not a cause.
It's normally for the tach to not work like that when you abruptly kill the power to the stock ECU.
-Ted
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