2000 SC300 rough idle and loss of rpms but not all the time.

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My sc300 will run fine one minute and will idle rough the next. Rpms will drop and the car acts like it will cut off but doesnt. Then it will go back to normal just to do it all over again. I dont have the slightest clue what the issue is. Any ideas are greatly appreciated. 

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Had the same issue. Ended up being the alternator. It goes slowly at first and makes the engine just behave weirdly.  I was checking idle air control valve and wires to ecu from various components.  But i gave up and eventually it got worse  and because the alt goes slowly it wasn't till it was downright bad that I caught it.  It usually goes around 200k miles...

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