Might start by checking for air getting into the fuel system. Replacing the hose from the return line on the injection pump with a clear one allows you to examine for air in the fuel.

Actually a valve adjustment check should be first. Mercedes recommended every fifteen thousand. On many of these cars the owners did not do it or assumed it had hydralic lifters.

If you find them tight. Adjust them and drive a few hundred miles and check them again. '

Also when you have the valve pan off check for the amount of chain stretch present. This is an easy check with information in the archives. The same with the valve adjustment check.

A timing chain stretched out will not cause your problem. Yet if stretched too much needs attention. Can cost the engine if it lets go. Also if you see no air in the clear plastic return line. Close it off temporarily. To see if that changes anything.