EGR Code P0400 - 96 C280 (140k miles)

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I've been searching posts trying to find the right info to fix my wife's 96 C280 (140k miles) but I've got some follow up questions.

The CE light is on and the engine code is the generic P0400. I ran the engine up to operating temp and she's idling smooth at about 750-800 rpm. I hooked up a hand vacuum to the EGR and when I hit about about 5-7 in. Hg vacuum the engine idle drops by about 50-100 rpm. I then ran the same test at 2000 rpm with about the same rpm drop when I get to about the same 5-7 in. Hg vacuum. When the rpms drop the engine runs a little rough, but it's not too bad (I imagine these Benz engines are well dampened?).

The incoming vacuum line to the EGR already appears to be rubber and in good condition all the way to the hard plastic line that runs somewhere else under the hood. I hooked the hand tester up to that incoming line and once I hit about 3000-3300 rpm I get a spike of vacuum pressure to about 20 in Hg VERY briefly (<2sec) then it drops back to 0. When I keep the rpms up in that 3500-4000 range I get a horribly rough shakeing.

So, a little advice please. What are my next steps?

Do these results mean the EGR valve is good? Should I replace the short rubber vacuum line to ensure it is OK, or the hard plastic one as well? Or should I just assume it's the EGR feeder tube (from the manifold to the EGR) and clean/replace it?

Thanks in advance! Any help is greatly appreciated.