I take it, this is the one with the 80K mi on it.
The only thing that comes to my mind would be an engine compression test.
If it turns out to be consistent, start pouring valve cleaner into your tank..
Sometimes, dirty valve seats can cause a problem at idle by leaking some. At higher RPMs, this may not be apparent or the higher pressure may help seal the valve(s) better.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert