[R129 SL] 1991 500SL stumbles after warm up

[Cliff Sadler]

All,
need your experience/expertise on this one.

I've researched all I can on this in the archives.

My car consistently does the following:
When starting cold, it feels like it's not running on all cylinders. Then
after a second, the idle rpms come up, and the car runs fine.  After a few
minutes, the car will begin to stumble again.  This has been intermittent
for months. Now it is guaranteed to stumble intermittently for a bit, and
then go to a clear performance loss, and vibration, like a cylinder has
dropped out of the mix. This remains until the car has sat for hours.

In an effort to remedy this, I have replaced the spark plugs (7 were clean
dry, and somewhat grey ash, 1 had some carbon, but not loaded up, or
fouled), distributor caps, and rotors. The car ran GREAT for about 48 hours,
then back to the same old thing. (sat for 3 days waiting for parts)

I checked all plug wires (didn't remove them though) to make sure the cap
ends were screwed on tight on both ends.

Now I'm starting to think that maybe the car has sludge in the gas tank, and
doesn't get starved until it's been moved for a bit, and it's not really
temp related.

Today, I'm going to start the car, and let it idle for a half an hour or so,
to see if it starts to stumble.

any other bright ideas?

TIA
Cliff