If you have a mechanical fuel pump, it may be a bent fuel pump rod. It will sound like a bad lifter or worse, like a loose rod. Could be a lifter that won't pump-up but you said you set the valves so that should have been detected. If you have to pull the pan off, check the edges of the rods and cylinder walls that they have clearanced enough for the 383. Change your oil, screen it for debris, eg. types of metals. Cut open your filter and check it too...

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