Greg, yours is doing nearly exactly what my SDL is doing. Mine starts out fine when it's cold, but as it gets to operating temp, it gets worse until it behaves nearly identically to yours, right down to the RPM ranges.

I've serviced everything in my system EXCEPT the DV's. I have the parts on order and will report back when I've done that job whether or not it made any difference.

At idle, mine has a soft miss (distinctly less than a dead cylinder) that goes away with any throttle at all. On the road, it's smooth as glass, but with that nailing right up to around 1300 RPM or so, then silence anywhere in the normal driving range.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]