Very likely that the fuel distributor is bad. I have had three of them go bad recently and the symptoms you have follow exactly. Smooth idle when first started, rough idle when warm, and normal performance when driving. Because of the cost I tried everything else first before I finally changed the fuel distributor. That was the problem. I opened the bad ones and sure enuf there were holes in the diaphram.

I just recently completely overhauled the intake manifold on my 560 for that very problem of rough idle. After re-sealing the manifold the idle improved but it did not completely cure the problem. The fuel distributor solved the rough idle. I am sure I had several vacuum leaks in the manifold. In fact I was sure of it. And I was hoping the seals were all I needed. I was not that lucky.

Let us know when you get the problem solved and what the cure was.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k