See my post in the SL forum. I had exactly the same symptom with a 2-3 flare and a highly trusted mechanic's recommendation for a re-build. He told me to drive it until it failed as the cost of a re-build was so high. Last week while poking around under the air cleaner shroud I discovered a loose vac fitting.

Reconnected line and the flare disappeared.

I also had a shifting problem on my 300SD with very HARD upshifts. In that case the issue was also a vac line - it was connected into an incorrect connector on that multiple vac connector thingy on top of the valve cover.

Your spring kit may be req'd but I've learned through the school of hard knocks that it could well be a vac issue and they can be annoyingly hard to locate!