If it's any consultation, my 4.5 does the same thing. Although the rough running appears to be from being to rich until the temp hits the magic 175. Car starts instantly, idle starts low with black smoke, slowly builds up and then drops off to normal as it appoaches the 175 mark. Then all is well until it cools off again.

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Litton
'90 420 SEL (sold)
'72 280 SEL 4.5
'98 ML320 (for sale)
'86 560SL
'05 Jeep Grand Cherokee Ltd (offroad in style)
'87 Chevy Blazer (AZ Pin Strips)