If you suspect the lifters, there's a FSM test you can do (Checking hydraulic valve clearance compensating elements)
http://www.ps2cho.net/downloads/MB%20CD/W124/Index/602_603/05Engine.htm
Although the site is for the W124, the engine is described as the OM603. The 603.960 is in the W124 and I believe the SDL has the .961. I'm not sure of the differences...
I'm not sure how to test the valves (the FSM seems to have the tests after the valves are removed), but, perhaps a compression test or leakdown test may prove more useful in figuring out which area to look at. The replacement FSM articles are in the above link.