Your lifters are hydraulic and they call them hydraulic valve compensation units. I hear that they don't often go bad, but can. They should be replaced with a valve job. Could be the cam. The units are something like $35 each (x 8) and I understand thsy are not too hard to change but you do have to pull off the valve springs and rocker arms. Be sure to keep the rocker arm with the same cam lobe. My $0.01

Steve
85 SL500 Euro