Sounds like something is very wrong with your timing. Very extreme timing chain stretch maybe?

Check all timing related items like put crank at tdc and check cam for timing, then make sure the pump is in there right at 24. Check for chaing stretch.
I beleive it is #1 and #4 that fire at only 4 degrees of movement and a little off on the timing can make a huge difference on the car and the way it runs.

Sorry I have no magic fix other than to say back up to where it was when running good then go through the steps you took when changing things. One of these must have caused the problem.

I'm also guessing it is a 300D...

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'99 S420 - Mommies
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'85 300SD Silver - Sold
'78 Ski Nautique