93 300SD degrading idle
 

The SD idles really nicely when started cold. In fact it's really easy to start when cold. Cold = 50-60F these days. At operating temp, I'll get to a red light and wait. As I wait, the idle gets lumpier and lumpier. RPM on the tach doesn't change but it feels like I'm on a horse. Get moving, get to the next light and the idle is better at first but degrades over 10-20 seconds to the point that I wonder if it's going to stall. AC compressor on or off doesn't change the idle. I can't isolate the lumpiness to a particular cylinder. Delivery valve holders are dry on the outside. Pulls fine from a stop and cruises nicely.

Also harder to start warm than cold. Not objectionable. When cold it seems to catch within the first crank revolution. When warm I have to hold the key for a couple of three seconds before it catches.

Any thoughts on what's going on? Rotational inertia straightens rods at speed but they bend at idle?

Thanks,
Sixto
93 300SD